Boss Spot HP can be enabled alongside Boss Randomization. Spot HP means that instead of scaling to the predetermined level of the dungeon, bosses will take their HP directly from the HP of the vanilla boss that sits there. There are different rules for determining HP splits if a multipart boss is involved, either originating in the slot or being shuffled into one. When a single-part boss replaces another single-part boss, it will simply take on the HP of the boss it replaced. For example, whether Yakra, Yakra XIII, or Masa&Mune appear in Heckran Cave, they will always have 2100 HP there.
Multi-part bosses are assessed for which of their parts are important - this usually means you're required to kill it (thus excluding, for example, the bottom half of Terra Mutant and Giga Gaia's hands), but exceptions such as Retinite's Core exist. The sum of these important parts makes up the HP total for these spots. If a single-target boss is shuffled in, its HP will be this number. If a multi-part boss is shuffled in, the HP total will be proportionally divided amongst the important parts. Then, any unimportant parts will have their HP calculated based on the original ratio of their HP to that of the important parts (exception: Mother Brain's screens will always have 1 HP).
Some bosses, like Rust Tyrano and Terra Mutant, have special rules for how their HP is calculated based on the Spot HP as well.
The vanilla boss is noted above each table.