While working with Canvas rubrics, I've found that RubricCriterion objects' `id` property values seem to be special.  Unlike most IDs, they are not integers.  Instead, they are strings and they start with an underscore ("_").  See the example in the documentation…

https://canvas.instructure.com/doc/api/rubrics.html#:~:text=A%20RubricCriterion%20object%20looks%20l...

Why is this ID value format so special?  Does the underscore indicate that it is to be joined to something else, like to the ID of the rubric in which it appears? Are these criterion IDs unique among all criterion, or only within the associated rubric?

Is the ID following the underscore always an integer?  That could lend itself to being an ID for the criterion when it is stored in another data source.