A manufacturer will 'demonstrate' crosswind landings on a new type ie the company test pilots will 'explore' the practical limits during test flying.
This limit will be the maximum that most companies would consider using due to liability issues. I think the 737 has been 'demonstrated' by Boeing at 40kts on a dry runway, and this indeed was the BA limit for captains (when I left last year, anyway). My current operator specifies 35kts dry.
There is no reason to expect that it would be 'impossible' nor unsafe to exceed this limit. It is just that Boeing will only 'guarantee' this product up to 40kts.
The limits are nearly always reduced for wet, narrow and contaminated runways. Normally co-pilots have lower crosswind limits. Also it is common practice to consider only the steady crosswind rather than the gusts in analysing the situation, although there is always a subjective input in that process!
I think that's about all I know. HIH
