A drawing layer is a simply a blank layer. It is made so that you may draw on these blank layers instead on items that are from the library.
Drawing layers are made for two reasons.
The first one is to discard your drawings quickly. This is useful if for instance you drew something, don't like it, and want to remove it: simply remove the drawing layer instead of undoing every drawing action.
The second one is that you will always have in your item list an untouched item from the library. This enables you to go back to it at any time if needed, or to compare the original item to your current work.
There are several ways to add a drawing layer to your character. The most straightforward is from the appropriate command:
|Tool bar||Menu bar||Shortcuts|
|> Add drawing layer||Character > Add item or drawing layer > Add drawing layer||Shift + I|
Nevertheless you shouldn't care about adding drawing layers manually. Game Character Hub does that for you: whenever you try to draw on an item, it automatically inserts a drawing layer for you to draw on.