To make sure only athletes who have been nominated are used in a particular date do the following steps. Otherwise the next time you add events or import/sync new/more members it will overwrite the nominations you have set up. This is because in a normal meet date, this is what you want.
1. Go to Event - Calendar
2. Then click on view for that meet date
3. Then scroll down below the event matrix to the button Meet Options
4. Then in here change the status from No to Yes and press submit. Note that when setting up your event/program if you choose the ""I'll nominate later" option, it will make this yes for you.