I've never encountered that acronym, but from what I've learned:
Usefulness: what a historian studying X can learn from the source
Can you see information relevant to X?
Are there facts, dates, and statistics? Descriptions of living conditions or whatever? Use quotes to back yourself up.
Note that an unreliable source can still be useful, as you can discern external information from bias that you recognise: for example, a Soviet newspaper article from 1951 written to praise Stalin can tell you about Stalinist propaganda and repression, even if all the content is rubbish.
Pretty much everything we get during exams is useful to some degree, you just have to figure out how and why.
Reliability: how much you can trust the information in the source
Who wrote it? What are their biases?
What are their qualifications and/or experiences that would make them an authority on the subject?
Who is it targeted towards? What kind of motive is behind it?
Compare it to your own knowledge of the subject matter: is the information right?
There's a lot more, but that's what I can remember off the top of my head.
