I'm a little confused... there shouldn't be a comma here. If anything, there should be a semicolon --> "...called Sam; it sat on the mat" because there are two phrases that could stand independently as sentences. This doesn't happen if there's just a comma there, unless there's a conjunction along with it (and, but, or, nor, etc.).
So I guess the technical term is a semicolon? Though that doesn't seem so technical...
Correct usage of a comma: The cat, called Sam, sat on a mat.
Or: The cat, who sat on a mat, was called Sam.
Or: Sam, a cat, sat on a mat.
Or: The cat, who was called by the marjority of unsaid persons as Sam, sat on the mat.
Or: Upon the mat sat a cat... No, forget that last one.
Love is friendship that has caught fire. It is quiet understanding, mutual confidence, sharing and forgiving. It is loyalty through good and bad times. It settles for less than perfection and makes allowances for human weaknesses. — Ann Landers