What is the difference between a collaborator and a committer?

A committer counts towards the billable Committers Total.
Collaborators can interact with your repo, but doesn't count towards your billable total. You can have as many collaboarators as you want.

Committers have these repo permissions

• push commits
• create new files

Collaborators can have the following repo permissions

• Issues: read, create, comment, edit, and attach files
• Design read, reviews: create, comment, edit, and attach files
• Releases: read, create, comment, edit, and attach files
• Wiki: read, create, edit
• Projects: create, edit

main accounts article.