Yes the parameters used to simulated the data. So if in my simulated data I have something like depth to ground water predicts tree productivity and I have depth to ground set at 0.5 per unit of productivity, my 50% uncertainty intervals should include 0.5 for that predictor. If not it’s likely my model is messed up.
Over here Stan simple model for pharmacokinetics @martinmodrak talks about another way to set priors if you are missing domain knowledge.