An assignment rubric is a scale used for assessing an assignment submission. It serves to further guide learners in the successful completion of their assignment tasks, as well as to help the reviewer to apply consistent standards to their assessment.
A rubric will not exist as an independent content object in those cases where it is trivial (i.e. for ungraded assignments, or if it's graded on a simple point scale), but in many cases a custom rubric will need to be authored and linked to an assignment.
The rubric, being an integral part of the guidelines for assignment completion (whether authored as a custom content object or not), should always be linked to the assignment description itself.