Spaces Panel
The 
Spaces panel provides an innovative feature to define areas within the PDF that makes storing and sorting markups easier and more efficient.
Go to Window > Panels > Spaces or press ALT+S to show the Spaces panel.
Spaces Panel Toolbar
The Spaces panel toolbar contains tools for adding, editing, manipulating, and deleting Spaces.
Add Space: Creates a new space.
Highlight Spaces: Toggles between showing and hiding existing Spaces.
Edit Spaces: Allows resizing and other editing of existing Spaces.