In professional football, a transfer is the action taken whenever a player under contract moves between clubs. It refers to the transferring of a player's registration from one association football club to another. In general, the players can only be transferred during a transfer window and according to the rules set by a governing body (fulfilling the requirements of FIFA, continental and national bodies regulating the purchasing and selling clubs). A negotiated transfer fee is agreed financial compensation paid from an interested club to the club that possesses the player's exclusive contracted playing rights. When a player moves from one club to another, their old contract is terminated whilst the player and their new destination club will both negotiate on new contract terms (or have earlier mutually agreed on the personal terms). As such, the transfer fee functions as financial compensation (paid to the club which possesses the existing playing rights) for the early mutually agreed termination of the contract of a professional football player. Transfer fees are contingent on the player’s current football abilities, future potential, duration of the existing contract, amount of future salary owed (within the remaining duration of the existing contract) and the willingness of clubs to agree on an economic equilibrium through supply and demand.