Adding on to www’s great reply:
Both lowest selection rank and lowest ATAR to receive offer are specific to the cohort and will differ each year, as this is dependent on demand and spots available. Using your example, a 98 selection rank won’t necessarily always guarantee you’ll get a spot in the Bachelor of Laws (apart from Monash Guarantee), if more people put this as a preference. It’s merely a guideline based on previous cohort(s) and you won’t find out what it was for your cohort until offers arrive and stats start to roll out.
Fortunately for most courses that don’t require any other criteria apart from ATAR/ selection rank, the lowest selection rank usually remains relatively similar, give or take some “ATAR points” in either direction, so the one on the courses are typically only a guideline, rather than a set bar, if that makes sense.