I think it is more to it than that. The index card only tells them that a seat is available. They also need to read other documents to find out which fares are

I think it is more to it than that. The index card only tells them that a seat is available. They also need to read other documents to find out which fares are available and match that to the seat. There are probably specific conditions for that fare that need to be applied and may require calling the agent or or someone else to confirm. If the seat is too expensive or the fare is unavailable, they need to reset the index card, pick a new one and repeat the process.