To report some error that need to be fix.
Used as an issue, to allow people to say something publicly to the team or for people willing to dev on the lib.
For bug/error that are really preventing you to use the lib and need to be fix as soon as possible, don't overuse it.
For merge request implementing a feature.
For merge request wich fix a bug.
For work that need to be done, to remember it and to show that it's planned.
For merge resquest that update the lib, like merging develop into master.
To ask something to be implemented but is not crucial.