In short
The answer first
Squad registration rules determine which contracted and registered players a club can list as eligible for a particular competition. The limits are competition-specific. In the Premier League model, for example, the senior list can contain up to 25 players with no more than 17 who do not meet the Home Grown Player criteria, while qualifying Under-21 players can sit outside that 25-player limit.
Key takeaways
A player can be employed by a club yet still need competition registration before appearing in that competition.
Squad limits are set by each competition and can differ between domestic leagues and UEFA competitions.
Home-grown status is a registration-history criterion and is not simply the same as nationality.
Youth-player exemptions can sit outside the main senior-list limit under specified rules.
Squad lists may be resubmitted or amended at windows or designated registration points under competition rules.
Transfer registration versus squad registration
A club first needs the player's underlying registration and contractual position to be in order. A competition can then impose an additional squad-list process that determines which players are eligible for that tournament or league period.
This is why a club can own a player's registration rights but leave the player out of one competition's submitted squad while using them in another competition if the other rules permit it.
The Premier League example
Premier League guidance currently states that a club's senior squad can contain up to 25 players and no more than 17 players who do not meet the Home Grown Player criteria. The remainder, if the club uses all 25 places, must satisfy the home-grown definition.
The home-grown test is based on registration with qualifying English or Welsh clubs for the required period before the age threshold; it is not a nationality test.
Why Under-21 rules matter
Qualifying Under-21 players can be eligible outside the Premier League's main 25-player list under the relevant seasonal age definition. That allows clubs to use academy and young first-team players without consuming the same senior-list places.
The exact birth-date threshold changes with the season, which is why a guide should explain the mechanism instead of permanently hard-coding one year's date.
Different competitions, different lists
UEFA competitions use their own registration rules and list structures, so a Premier League squad cannot simply be copied into a European competition. Cup competitions and domestic associations can also set separate eligibility requirements.
When a player is described as 'not registered', ask the next question: not registered for which competition and for which registration period?
Source check
Sources and further reading
KickVerse uses primary governing-body material where a rule or competition format has an official source. Analysis terms can vary between data providers, so provider-specific definitions are identified rather than presented as universal laws.
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