http://www.fivb.org/EN/Referee…ules_2017-2020-EN-v06.pdf
19.1 DESIGNATION OF THE LIBERO
19.1.3 The Libero on court is the Acting Libero. If there is another Libero, he/
she is the second Libero for the team.
Only one Libero may be on court at any time.
19.3.2 Libero Replacements
19.3.2.2 The regular replacement player may replace and be replaced by
either Libero. The Acting Libero can only be replaced by the regular
replacement player for that position or by the second Libero