The rule, article 124.3 in the federation’s regulations, allowed clubs to make an emergency signing outside of the transfer window as a replacement for an injured player.
The long-standing rule was available for all clubs inside the Spanish league system, and allowed them to sign one player from within the Spanish league system (or a free agent) to play domestic competitions.
Barcelona were the first to use it when they got Braithwaite from Leganés after Ousmane Dembélé went out for the season.
Celta Vigo followed suit, using the same rule to land winger Nolito after their backup keeper, Sergio Álvarez, went down injured.
The rule was deemed unfair by the media as teams had no recourse when a player’s release clause was triggered. Leganés were relegated, in part because they could not themselves use that rule to replace Braithwaite - their main striker - as they themselves had no long-term injury.