A TA PO or Soldier is attached to a UOTC for Officer Training - he completes the UOTC modules (MTQ 1-3) during his time with the UOTC. He/She is still a TA soldier, and is required to pass relevant MATTs tests, and is paid TA wages and bounty. After finishing MTQ3 he can attend RMAS, and upon passing, receive his commission and complete MOD 5 of the TA Officer Training Scheme - Becoming a Grp A Officer.
He is able to complete TA training with the unit which is sponsoring him (especially MATTs), and it is them that pay him/her and handle the majority of admin.
A good option to give to a student in the TA who's leaving for a far away University - i.e. he trains with UOTC in term time, doing his officer training, and then he'll complete MATTs training with his Unit back home in holidays, and will probably be able to do their annual camp. He'll receive a TA bounty - instead of the Â£150-180 OTC OCdts receive. On certain occasions the TA will pay for a rail warrant for him to attend certain weekends, so he can come back home to do a MATTs w/e, Remembrance Parade etc (and whilst they've paid for him back home he can have sunday lunch with his family or 'see' a girlfriend back home, before going back up to Uni).
In some respects it's another option for people who can't do Bde TAPO weekends. However, although UOTCs can offer excellent tuition, in my opinion they're really on a lower level than the required levels MOD 1 - 3 that Bde RTCs expect. Hence, the TAPOs (OCdts through Bde RTC Officer training) are generally better prepared for MOD 4 at Sandhurst than the TASOs (OCdts who did MTQ 1-3 with an OTC). This is no comment on the quality of officers either churns out - just a comment on the methodology of training the Officer has experienced - and indeed, a some of the best YOs I've known came from UOTC. But you should really seek someone with a better experience and authority than my impressions, opinions and interpretations.