When I availability check somebody, I am seeing if they are free to film on certain days. The clients often ask questions such as filming locations and rate of pay, so it is important that I know about the productions. It is important to let the clients know that it is an availability check, and they are not being booked as of yet. If they are free then I make a note on a document, paper, or our company digital dairy spreadsheet. If there are 20+ people needed on a filming day, I would probably just availability check via email.