Well actually there’s a bit more to that:
The BIOS loader can boot either 32-bit/64-bit Haiku.
The 64-bit UEFI loader can boot only 64-bit Haiku. (and it has been around for a long time so it’s quite stable)
The 32-bit UEFI loader can boot only 32-bit Haiku. (and it has been added recently so it should be still considered somewhat experimental)
I have a development branch that contains the code for a ‘mixed-mode’ UEFI loader but it hasn’t been merged. Theoretically it should be possible to load both combos with that, i.e. boot from 64-bit UEFI to 32-bit Haiku or from 32-bit UEFI to 64-bit Haiku (in the rare case of a 64-bit CPU with 32-bit UEFI firmware)