Do you mean Constant Framerate and Variable Framerate?

Sorry, what I meant is under the Video tab in Handbrake, there's the Framerate (FPS) setting and below that are 2 bubble options. Due to the profile Handbrake starts up to (I haven't gotten around to setting up my own preferred profile just yet), the framerate is set to "30", and the two bubble options are "Constant Framerate" and "Peak Framerate".

However, when I change the framerate from "30" to "Same as source", the two bubble options change to "Constant Framerate" and "Variable Framerate".

What I wanna know is if you know why that changes? And if so, what does it mean?

This is not correct. MKV and MP4 are just container formats. The stuff that actually makes up the size is what goes into those containers (the video stream, audio stream, and to a much lesser extent the subtitle files, chapter markers, and any other metadata).

facepalm Holy bicycles, I knew that. Seriously, I did. I don't know what the fuck was going through my head to make me forget that. Lol. Sorry!

Then you should not use Handbrake, unless you plan to keep two copies of your media. MakeMKV should remux the blu ray discs directly into MKVs (I’ve never used it to do blu rays, but I have used it with DVDs to do this job). If you do anything to your media with Handbrake, it’s not really an archive anymore, it’s just a good copy. For many people this tradeoff is worthwhile because if you encode it at high enough settings you won’t get any visual loss but you will get a much smaller and more easily manageable file. But I would only call it an archive if it’s untouched from the source. That’s what the term “remux” implies: Taking the original video/audio stream contents and putting them into a new container such as MKV.

This is all true. I think rather "archiving" may have been a poor choice of word on my part, though it is partly accurate. I do think of this as an archive, but not so much that it needs to be an exact 1:1 ripping. As long as it is perceptually the same audibly & visually, it is largely good enough. I guess by "archive", I mostly meant as a backup; I didn't necessarily mean it as archival-level quality. Sorry about the confusion! That was on me!

Edit: a word