IMCS stands for Index Mast Check System.

By Googling say Windsurfer, Masts, IMCS you will quickly find mountains of technical reading matter. Enough to keep you
going for days !

Generally it's about the way a mast curves under a central load test when it's ends are supported. Some bend more than
others, some less. Some bend more in the top third, some less etc, etc.

You then relate that IMCS# back to a certain sail type or manufacturer.

As I said you'll find plenty of further explanations once the Google search is done.

Basic version:

IMCS is Stiffness relative to length.

Reason: before IMCS, masts were listed by stiffness. But if you made the same mast longer it would rate as floppier when in fact it was not...... it was the same but there was just more of it.

So now we have IMCS so a 400/19 is same as a 430/21.

That's all you need to know.

However bend curve... wellll geez ............

mast/sail compatibility topic on its own
picking the right sail for the conditions

my buddy says "bring all your stuff to the beach"
then change out sails n boards as required
he spends more time pickin n choosin than i
i am usually on the water within 20 minutes
for him double that
and he will change out more than i
i think i will be satisfied more easily than he
even if i am not planing, i can have FUN
if i am OP/overpowered - no issue - tighten it up and learn survivor mode
TOW = FUN

This is my "slightly" exaggerated guide to mast curves.

Use www.unifiber.net/masts-selector to match sails to masts.

Wave and freeride sails are a little more tolerant of mast curve but race sails can be unforgiving. But try and rig a Gaastra on a Neil Pryde, as I did when starting out, and you'll see an ugly outcome.

Any chance you get, bitch and moan to shops, manufacturers and pros - we need a standard mast curve!