This is the History and Mark-up tab; as suggested by the name, it contains the History and Mark-up windows.

The History window shows your editing history. It allows you to undo and redo changes in a visually intuitive way. It also provides information such as the kind of change that was made, the compression state of the undo buffer containing the change, and where the buffer is stored (RAM or disk).

The Mark-up window shows the Mark-up of the current layer. Visit Mark-up Attachments for more information on Layer Mark-up data.