Classic Match is a friendly game mode in FIFA and FUT which follows the classic / official rules of football.
By playing the Classic Match mode you will follow the regular rules of football and there will be no any special rules.
Substitutes
This game mode will allow you to substitute 3 times unless you customise it in the Game Settings (Kick-off mode).
How to Play Classic Match Mode
Survival friendly mode is available in FUT 21 > PLAY tab > FRIENDLIES (Couch Play / Play Online / Play A Friend).
In FIFA Kick-off mode, it is available under House Rules mode.