When should the libero replacement occur?

The libero replacement should occur before the start of each set after checking lineups. This process allows the officials and teams to verify that the libero is properly designated and that their participation aligns with the established rules. This pre-set confirmation is critical because the libero has specific roles and limitations during play, including not being allowed to serve, attack the ball above the height of the net, or block. By confirming the libero’s status prior to each set, it ensures that all teams understand the player dynamics and maintain compliance with the regulations governing the libero's role throughout the match.

In contrast, allowing libero replacements at just the beginning of the match, during stoppages of play, or only during timeouts would not provide the necessary clarity and structure that the protocol is designed to maintain. This structured approach helps to prevent any confusion or disputes that could arise regarding the libero's participation as the match progresses.