The characters owned by Crypton, plus Yowane Haku (who was an Internet Creation whose main creator basically gave Crypton "do whatever" privs IIRC) are classified as "Miku and Friends"... In part because they're now on a newer software than Yamaha's VOCALOID.
UTAU is a different vocal simulation software, and its big thing is anyone can create voices for it - which led to a boatload of characters ensuing very quickly.
In the fandom, it's common to treat the characters as one huge cast. In official marketing which Crypton has long ruled the roost on, not so much obviously
the rich and varied tapestry of vocaloid culture is fascinating when you consider how Hatsune Miku has about as much canon characterization as Mr. Clean
In terms of internet vocaloid fandom, Gakupo snd Gumi are often grouped with the Crypton gang despite not being owned by Crypton, mostly bc they came out around the same time
- the Crypton Six (so, kaito, meiko, miku, rin, len and luka) belong to Crypton, who have split from Yamaha. They're not associated with Vocaloid anymore. - Any other characters outside of Yamaha were developed by separate companies. Their voices use the Vocaloid engine, but their characters belong to the original company.
originally, people started attaching it to people's screennames as a show of respect, but as the fandom expanded and got bigger, widespread use of it fell out of fashion
or like, they'd come up with a cutesy nickname for them that described their musical aesthetic and attach the "-p", so their "-p" name basically meant "the producer who makes this kind of music"
Anyone who isn’t the Crypton 6 is still a vocaloid
Technically they aren’t even the original vocaloids but that’s a whole other thing
- Any other characters outside of Yamaha were developed by separate companies. Their voices use the Vocaloid engine, but their characters belong to the original company.