A physical therapist aide position? In general, I would say business casual. No jeans or the like, but also no high heels or other impractical pieces. It's probably best to follow the "Dress for the job you want" advice and model yourself after the PTs. Other clinics I've been in require the staff to wear srubs, but they would tell you that upfront. A DPT program orientation session? To be on the safe side, I would follow the same as above; you want to make a good impression but not look overdressed. A DPT program interview? Your nicest professional suit. A physical therapist interview? Same as above.