Maximum demonstrated crosswinds are published in the AFM. Generally, this is taken as guidance for crosswind operations, and unless the manufacturer specifically discusses wind gusts their application is open to operator / crew judgment.
Where a crosswind has been found to be limiting, it is published in the AFM as a limit (CS 25 AMC 25.1581 para 6). There does not appear to be any ruling on how wind gusts are published and hence 'legally' interpreted, with the exception of HUD certification (CS 25 AMC 1 25.1329 para 14.1.4.1).
EASA ref (FARs similar):
CS 25 Large Aircraft, Amendment 5.