RoboFont can handle layers inside a single UFO. The idea is to have it very flexible. Some will use it as a way to separate different stages of a font. Others will use it as way to collaborate and other others will use it like multiple master. 

Users can add, delete, reorder layers in the layer inspector pane.

There is no limit on the amount of layers in a glyph. 

Layers names are ASCII without spaces, slashes and semicolons.

Layers can have any color. Used to be displayed in the background of the glyph editor.

Read more: Inspector