Spaces tab provides an innovative feature to define areas within the PDF that makes storing and sorting markups easier and more efficient.The Spaces tab provides an innovative feature to define areas within the PDF that makes storing and sorting markups easier and more efficient.
Go to View > 
Tabs > Spaces or press ALT+S to show the Spaces tab.
Spaces Tab Toolbar
The Spaces tab toolbar contains tools for adding, editing, manipulating, and deleting Spaces.
Expand All: Expands all pages and nested areas to show all the Spaces defined in the document.
Collapse All: Collapses all pages and nested areas to show only the page labels.
Highlight Spaces: Toggles between showing and hiding existing Spaces.