Hi,
I’ve looked into this alot as I was very confused. Basically, 9 times out of 10 when a surgeon says unders they are in fact partials (or duel plane), it is very rare now a days for the implants to be completely under the muscle from top to bottom of the implant. When you have partials, only around half of the implant is under the muscle and the rest is free.
Hope this helps lol