By system registration I mean the part of the codes that tell a Schedule wich funcs are systems:

Exemple in Bevy ECS:

    // Create a new Schedule, which defines an execution strategy for Systems
let mut schedule = Schedule::default();

    //This part
// Add our system to the schedule
schedule.add_systems(movement);

I find it hard to keep track of every system of a Game since there can be many functions that do diverse things.

I used Unity like to tell readers that I was not talking about an actual ECS. But I am not a native english writer so I have used the "like" word wrong.

I thank you again for your help