Full board at my barn is feed/turnout/stall mucking.
Training board (and extra $400 or so) is 5 rides per week, any combination of lessons or trainer rides you choose. If you are not riding the trainers is always the one who rides, no assistants/students riding the horse. The schooling rides can be short 30 min or sometimes they will take one of the training horses for a hack/trot set school (eventing barn).
I will say that I have never seen anyone other than the trainer ride the training horses. There is a list of who needs to be worked each day and each horse gets tended to by the trainer. I have a flexible enough job that I can pop in during the day (and have for various reasons) and things are always going as listed on the schedule. If something has to change due to an emergency, the owner gets a text message/call explaining that the horse wasn't ridden and options are discussed.
There are trainers who will do what they say, properly, because they want the best for their pupils and horses. Find one of those - they are worth their weight in gold.