A franchise squad is assembled at auction but confirmed by registration. The date on which that list closes is what actually determines who can play.
Registration converts a contract into eligibility
Winning a player at auction creates a contract. It does not by itself make the player available, which requires the club to lodge the name on the league's list within a stated window.
The list is checked against overseas quotas, squad-size limits and the purse. A player who breaches any of them is rejected regardless of the contract behind him.
Because the check happens at registration rather than at auction, clubs sometimes discover a compliance problem after they have committed money.
The deadline exists to protect the table
If squads could change freely during a season, a team eliminated early could effectively lend players to a contender, and results earlier in the table would be inconsistent with results later.
A closed list means every team plays the same competition throughout. Sporting integrity arguments almost always reduce to that comparability.
The same logic explains why the deadline usually sits before, rather than after, the opening match.
Replacements are an exception with conditions
Injury and international withdrawal would otherwise leave a club short through no fault of its own, so rules permit named replacements under defined circumstances.
The conditions are narrow. A replacement typically requires medical documentation, must fit the same category as the departing player and must be approved by the league rather than announced by the club.
Price is usually capped at or below the original player's value, which stops a replacement being used to upgrade a squad mid-season.
Availability is a separate question from registration
A registered player may still be unavailable, because his home board controls when he is released. Registration establishes eligibility, not attendance.
Clubs therefore track two calendars: the league's registration dates and the international schedule that determines who will actually arrive.
Squads are built with that in mind, which is why teams carry more players in a given role than a single match requires.
The deadline shapes auction behaviour
Knowing the list closes early, clubs buy for the whole season rather than for the opening weeks. A player available for half the tournament is priced accordingly.
It also pushes clubs to buy depth in the categories where replacement is hardest to arrange, since those are the positions where a late loss cannot be repaired.
Registration looks administrative and is in fact the point at which a season's competitive shape is fixed. Everything after it is management of a squad that can no longer be rebuilt.

