Used as an adjectival noun meaning something is private, individual, or tailored to a specific person. Often seen in compound terms like パーソナルスペース (personal space) or パーソナルトレーナー (personal trainer).
This is a very personal matter, so I don't want to talk about it much.
I have a personal trainer at the gym.
This is a very personal matter.
To provide the service in a personalized way.