Goal: I want to update some Content Items in the Batch Editor, only a sub selection of which have a specific layer assigned.

1. I select ten items for batch editing. 
   Under Metadata, I see an overview of Assigned Layers and Available Layers for the ten selected items. 
   In the Assigned Layers section, each layer shows the number of items to which it is assigned. In this case, six of the selected items have the Media usage information layer applied, and four do not. 
   I select Media usage information to see which of the six items have the layer applied.
2. I select Media usage information to see which of the six items have the layer applied.
3. I then select Assign layer to 4 content items to assign this layer to the remaining items in the Batch Editor. 
   Using the Ctrl key, I select two of the four items. The number on the Save button automatically changes from 4 to 2 to indicate the selected content items.
4. I select Save.
5. I select Metadata to get an updated overview of layers for the ten content items in my selection. Now, the Media usage information layer displays 8, as this layer has been applied to two of the remaining four items.