diff --git a/src/ol/layer/Group.js b/src/ol/layer/Group.js
index 33afeb902c..611ed41295 100644
--- a/src/ol/layer/Group.js
+++ b/src/ol/layer/Group.js
@@ -21,8 +21,10 @@ import SourceState from '../source/State.js';
  * @property {boolean} [visible=true] Visibility.
  * @property {module:ol/extent~Extent} [extent] The bounding extent for layer rendering.  The layer will not be
  * rendered outside of this extent.
- * @property {number} [zIndex=0] The z-index for layer rendering.  At rendering time, the layers
- * will be ordered, first by Z-index and then by position.
+ * @property {number} [zIndex] The z-index for layer rendering.  At rendering time, the layers
+ * will be ordered, first by Z-index and then by position. When `undefined`, a `zIndex` of 0 is assumed
+ * for layers that are added to the map's `layers` collection, or `Infinity` when the layer's `setMap()`
+ * method was used.
  * @property {number} [minResolution] The minimum resolution (inclusive) at which this layer will be
  * visible.
  * @property {number} [maxResolution] The maximum resolution (exclusive) below which this layer will
diff --git a/src/ol/layer/Heatmap.js b/src/ol/layer/Heatmap.js
index 90c0436e84..7e1e3389e1 100644
--- a/src/ol/layer/Heatmap.js
+++ b/src/ol/layer/Heatmap.js
@@ -18,8 +18,10 @@ import Style from '../style/Style.js';
  * @property {boolean} [visible=true] Visibility.
  * @property {module:ol/extent~Extent} [extent] The bounding extent for layer rendering.  The layer will not be
  * rendered outside of this extent.
- * @property {number} [zIndex=0] The z-index for layer rendering.  At rendering time, the layers
- * will be ordered, first by Z-index and then by position.
+ * @property {number} [zIndex] The z-index for layer rendering.  At rendering time, the layers
+ * will be ordered, first by Z-index and then by position. When `undefined`, a `zIndex` of 0 is assumed
+ * for layers that are added to the map's `layers` collection, or `Infinity` when the layer's `setMap()`
+ * method was used.
  * @property {number} [minResolution] The minimum resolution (inclusive) at which this layer will be
  * visible.
  * @property {number} [maxResolution] The maximum resolution (exclusive) below which this layer will
diff --git a/src/ol/layer/Image.js b/src/ol/layer/Image.js
index 38c69f79c4..1ddf7bbb3f 100644
--- a/src/ol/layer/Image.js
+++ b/src/ol/layer/Image.js
@@ -11,8 +11,10 @@ import Layer from '../layer/Layer.js';
  * @property {boolean} [visible=true] Visibility.
  * @property {module:ol/extent~Extent} [extent] The bounding extent for layer rendering.  The layer will not be
  * rendered outside of this extent.
- * @property {number} [zIndex=0] The z-index for layer rendering.  At rendering time, the layers
- * will be ordered, first by Z-index and then by position.
+ * @property {number} [zIndex] The z-index for layer rendering.  At rendering time, the layers
+ * will be ordered, first by Z-index and then by position. When `undefined`, a `zIndex` of 0 is assumed
+ * for layers that are added to the map's `layers` collection, or `Infinity` when the layer's `setMap()`
+ * method was used.
  * @property {number} [minResolution] The minimum resolution (inclusive) at which this layer will be
  * visible.
  * @property {number} [maxResolution] The maximum resolution (exclusive) below which this layer will
diff --git a/src/ol/layer/Layer.js b/src/ol/layer/Layer.js
index ceb8c186b3..1d5f68bc81 100644
--- a/src/ol/layer/Layer.js
+++ b/src/ol/layer/Layer.js
@@ -18,8 +18,10 @@ import SourceState from '../source/State.js';
  * @property {boolean} [visible=true] Visibility.
  * @property {module:ol/extent~Extent} [extent] The bounding extent for layer rendering.  The layer will not be
  * rendered outside of this extent.
- * @property {number} [zIndex=0] The z-index for layer rendering.  At rendering time, the layers
- * will be ordered, first by Z-index and then by position.
+ * @property {number} [zIndex] The z-index for layer rendering.  At rendering time, the layers
+ * will be ordered, first by Z-index and then by position. When `undefined`, a `zIndex` of 0 is assumed
+ * for layers that are added to the map's `layers` collection, or `Infinity` when the layer's `setMap()`
+ * method was used.
  * @property {number} [minResolution] The minimum resolution (inclusive) at which this layer will be
  * visible.
  * @property {number} [maxResolution] The maximum resolution (exclusive) below which this layer will
diff --git a/src/ol/layer/Tile.js b/src/ol/layer/Tile.js
index 1782d8c329..1fa5dcd75c 100644
--- a/src/ol/layer/Tile.js
+++ b/src/ol/layer/Tile.js
@@ -13,8 +13,10 @@ import {assign} from '../obj.js';
  * @property {boolean} [visible=true] Visibility.
  * @property {module:ol/extent~Extent} [extent] The bounding extent for layer rendering.  The layer will not be
  * rendered outside of this extent.
- * @property {number} [zIndex=0] The z-index for layer rendering.  At rendering time, the layers
- * will be ordered, first by Z-index and then by position.
+ * @property {number} [zIndex] The z-index for layer rendering.  At rendering time, the layers
+ * will be ordered, first by Z-index and then by position. When `undefined`, a `zIndex` of 0 is assumed
+ * for layers that are added to the map's `layers` collection, or `Infinity` when the layer's `setMap()`
+ * method was used.
  * @property {number} [minResolution] The minimum resolution (inclusive) at which this layer will be
  * visible.
  * @property {number} [maxResolution] The maximum resolution (exclusive) below which this layer will
diff --git a/src/ol/layer/Vector.js b/src/ol/layer/Vector.js
index 8aa2cbe596..fce9e625b3 100644
--- a/src/ol/layer/Vector.js
+++ b/src/ol/layer/Vector.js
@@ -14,8 +14,10 @@ import {createDefaultStyle, toFunction as toStyleFunction} from '../style/Style.
  * @property {boolean} [visible=true] Visibility.
  * @property {module:ol/extent~Extent} [extent] The bounding extent for layer rendering.  The layer will not be
  * rendered outside of this extent.
- * @property {number} [zIndex=0] The z-index for layer rendering.  At rendering time, the layers
- * will be ordered, first by Z-index and then by position.
+ * @property {number} [zIndex] The z-index for layer rendering.  At rendering time, the layers
+ * will be ordered, first by Z-index and then by position. When `undefined`, a `zIndex` of 0 is assumed
+ * for layers that are added to the map's `layers` collection, or `Infinity` when the layer's `setMap()`
+ * method was used.
  * @property {number} [minResolution] The minimum resolution (inclusive) at which this layer will be
  * visible.
  * @property {number} [maxResolution] The maximum resolution (exclusive) below which this layer will
diff --git a/src/ol/layer/VectorTile.js b/src/ol/layer/VectorTile.js
index 17559cbd6f..204cc4a81d 100644
--- a/src/ol/layer/VectorTile.js
+++ b/src/ol/layer/VectorTile.js
@@ -35,8 +35,10 @@ export const RenderType = {
  * @property {boolean} [visible=true] Visibility.
  * @property {module:ol/extent~Extent} [extent] The bounding extent for layer rendering.  The layer will not be
  * rendered outside of this extent.
- * @property {number} [zIndex=0] The z-index for layer rendering.  At rendering time, the layers
- * will be ordered, first by Z-index and then by position.
+ * @property {number} [zIndex] The z-index for layer rendering.  At rendering time, the layers
+ * will be ordered, first by Z-index and then by position. When `undefined`, a `zIndex` of 0 is assumed
+ * for layers that are added to the map's `layers` collection, or `Infinity` when the layer's `setMap()`
+ * method was used.
  * @property {number} [minResolution] The minimum resolution (inclusive) at which this layer will be
  * visible.
  * @property {number} [maxResolution] The maximum resolution (exclusive) below which this layer will