Protobi has a new Notes feature that may help you coordinate revisions with your team and clients.
For any element, hover over the header to see the notes icon in the upper right:
Press the icon to create a note to the element:
If an element has at least one note, the icon will appear with a count. 
Press the icon to see all notes attached to the element and optionally add more or delete notes that are resolved.
in the upper right corner an icon shows the count of all elements with at least one note. Press that to bring up a list of all notes in the project. Double click on any note to navigate to the associated element.
How you use notes is up to you. We use them ourselves to suggest revisions, and as a convention expect that they will all be cleared by the time the view is ready for the client to review. You can add more notes to form a discussion, attach background info, or other things you can imagine.
Right now notes are part of the project so you have to save the project for them to be visible to colleagues and clients. And as a result notes can be seen or created by users with any level of permission on the project.
We're working to separate that so that multiple team members can save and see notes simultaneously without saving the project itself, and so that end clients can attach their own notes even if they can't edit the project directly.
Please contact us with your feedback and suggestions at support@protobi.com.