What could result in an illegal substitution during a match?

An illegal substitution occurs when the rules governing substitutions are not followed, leading to a violation. Substituting a player who is not on the official roster is invalid because the rules specify that only those players who are formally part of the team's roster can participate in the match. This is crucial for adhering to tournament regulations and maintaining the integrity of the game. If a player is not listed, their involvement can compromise the competitive balance and fairness that the roster system is meant to ensure.

In a match, all players who participate must be accounted for on the official roster to ensure proper tracking of eligibility and substitutions. This prevents confusion over player identities and maintains accountability regarding player participation which is fundamental to the officiating standards in volleyball.