"Up on that" is an idiomatic phrase used to refer to one's knowledge or understanding of a particular subject. Synonyms for this phrase include "familiar with," "well-versed in," "informed about," "up-to-date on," and "knowledgeable about." These synonyms suggest that the person in question has a good grasp of the subject matter and is able to navigate it with proficiency. Other synonyms for "up on that" might include "experienced in," "skilled at," "versed in," or "proficient in," all of which convey the idea of expertise and mastery. Whatever the synonym used, it should emphasize the idea that the person is knowledgeable and confident in their understanding of the topic at hand.