volountary - status is volountary (in theory) you can leave if you want.
sectioned - sectioned for compulsory assessement and / or treatment under the mental health act. if you want to leave... you need to appeal that in court.

not everyone has been to hospital for BPD.

but not only that...

people don't go to hospital for BPD / schizophrenia etc.
people go to hospital typically because they are a danger to themself and / or others and they figure a little time to stabilize them on medications (most typically) will benefit them.

it is controversial whether an inpatient environment is helpful or harmful for people with BPD.