As stated, if ubiquitination is mentioned on the MCAT its probably going to be in reference to general protein degradation. There are other functions of protein ubiquitination that you can look up if your interested.
Histone ubiquitination has been associated with transcriptional activation and polar body formation. However, I don't think there is a general consensus on the function(s) of histone ubiquitination. If your really interested You can do a pubmed search and read a review article on the topic 😉 .