The Collaborator View is when the user is on a paper as a collaborator. There are a few differences between regular paper Focus Mode and Collaborator View Focus Mode.

The biggest difference is that when it comes to Return Section, which is usually at the top of the paper, on Collaborate View Focus Mode the same section is at the bottom of the paper.

In regular view, the Return Section is on the top of the paper:

For more information about Focus Mode, please check here