When players are part of your academy you can play them in matches. Whilst a player is below the age of 21, you will have the option of signing him for your squad or releasing him. If you want to sign him, there will be a small "signing-on" fee, which is used for prize pools. Once an unsigned player reaches 21 they become a free agent and can be signed by any club. If you release him, any other club will be able to sign him via an auction.