I just noticed that there's a line on the page where you add modifiers to homebrew content called "Additional Bonus Types." It has three options in it. Attuned Item Count, Proficiency Bonus, and Spell Level. What exactly is this line for, and what do each of these options actually do?

I'm really curious about the answer as I just implemented a "Proficiency Based Attunement" rule a game that I'm running, and I want to let the Artificer not have to worry about keeping track of their additional attunement slots from their class. That is a problem because the way of setting attunement slot count on this site forces a maximum of 6 slots, which I am slightly annoyed by. If this offers a way around that maximum, I would be extremely happy about it.

I should probably let someone who actually knows answer first, but my best guess is that it does exactly what it says.

For example, the Summon spells deal damage along the lines of 1d6 + Spell Level. (As used when upcasting a spell.)

Artificers, at later levels, also get bonuses according to the number of attuned items.

It lets you add one of those 3 non-fixed values on top of the regular Fixed Value (and ability modifier when applicable) to whatever Modifier you're using. It's relatively new, and I'm not sure if they're limited to particular modifiers, you'll need to test and see.

Edit: So if you're wanting to have attunement slot counts based on Prof, you could make the modifier Set - Attunement Slots, Additional Bonus Type: Proficiency Bonus

Edit2: Oh. I just tried doing Set - Attunement Slots, and it's not using PB. Oh well.

It honestly depends on which Modifier is used. Sometimes it applies the previous bonus to the things in that line, sometimes it applies the things in that line to the modifier, sometimes it doesn’t do anything at all.