Sounds like they're actually similar to archetypes and I'm just dumb, tbh, but basically in 3.0+ D&D there were classes you could multiple class into without multiple penalty if your character met specific qualifications (different for each prestige class, usually ability score minimum and knowledge of a feat, spell, or spell level, but sometimes specific race or language or whatever). These classes were usually much more specialized and specific than the general core classes, but also gave your character great powers and flair in that specific niche. Or at least that's the idea when they were well-implemented, which was not always the case, and prestige bloat is often cited as one of the worst parts of 3.0+ as nearly every single sourcebook would include at least a couple new ones (but I never saw this a problem, personally).