Whether you are an actual or reserve participant is determined by the doctor (within the context of the quality of the trial). This happens after check-in at the clinic, when everyone has gone through a final check. If you are a reserve participant, you will stay in the clinic until administration of the first dose, just to be sure. For this you will also receive so-called 'compensation for reserve participants' (in addition to your travel allowance). This compensation for reserve participants is stated in the ICF that you sign at the time of your screening visit. In most cases you can move on to the next trial group (depending on the start time, you may have to undergo another medical examination) or participate in another clinical trial.