Low Rank and High Rank confusion!

I'm sorry for making this post, I hope it's not too off topic but I can't seem to find a concise explanation about how low and high rank work in this game? Maybe that's because we don't know enough or maybe I suck at searching through everything we know but I was trying to find some sort of simplified explanation. If anyone can give me a quick explanation I'll be quick to delete this thread soon after.

It seems in my haste I completely forgot to mention that I was primarily trying to understand if we knew which monsters that we knew of (including all datamined monsters) are low rank or high rank. I'm mostly curious because in my friend circle we remark on how simple Rey Dau's kit was in the beta but figured it was related to low rank or something adjacent of it. Made me realize I wasn't really understanding where monsters would start to become high rank. I think I read something like all apexes have a HR counterpart which makes sense, they're sort of VIP as it were. Was just curious if we knew of any other HR or even monsters that are only HR!