Without getting overly complicated, yes, baby pak choi is just an immature plant.

Thanks. I’ve since seen that one can buy varieties of the ” baby, ” but the regular will be fine.

You can pick a standard pak/bok young, while it's a baby. And call it baby. Most micro or baby green mixes and the like contain standard more often than a true baby variety.

There are also baby or tiny varieties of pak/bok. I grow a variety of baby pak/bok that is a true tiny (Hedou), and only grows to a few inches tall/wide. I use that seed for square inch gardening in trays. Mostly in winter. I also grow a standard pak/bok in the garden, and wouldn't try small growing it in trays like I do Hedou. It would get too big for the tray before being actually big enough for harvesting properly.

If you want to grow pak/bok and just be able to trim out some baby leaves, I would just use standard seed. If you want to grow actually small, I would use a tiny seed.