Automatic Substitution

I dont know really now how works and how to set automatic subsitution before match .
Can anyone explain that part of the "top eleven~"?

Go into the team tab, then select the Automatic Substitutions tab.
Select your substitutes for each position and drag them to the window on the right.
In my example there are only 2 substitutes because I'll be watching my games today.
Automatic substitution only works when you're not watching the game.
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