What is the expected behavior in Mantra for reviewers taking ownership of 'Files for Review'? In TMS it's clear, one just clicks on it and takes ownership. In Mantra it's not so clear as we see some files for review greyed out?

What is the expected behavior in Mantra for reviewers taking ownership of 'Files for Review'? In TMS it's clear, one just clicks on it and takes ownership. In Mantra it's not so clear as we see some files for review greyed out and other reviewers can no longer access them in the same group?