PDF layers are, like layers in many CAD programs, a way of organizing the content in a document. Think of layers as a switch that can be toggled on or off to control the visibility of everything on that layer. Layers can hold content and markups; markups can be moved from one layer to another. Markups can be flattened to a layer, unflattened from that layer, and copied and pasted while retaining their layer. Layers also appear in the Markups list to allow organizing and processing markups by layer.
Most of Revu's layers functionality is accessed through the Layers panel. For an overview of the Layers panel and the commands available there, see Layers Panel.
Note: Unlike in most graphics programs, layers in PDFs do not affect the display arrangement of content. To arrange the relative position of overlapping markups, see Reordering Markups.