sleeving is when they remove your oem sleeves and install new ones.
instead of seeing the gap between the sleeve and the block, they actually install a sleeve that allows you to go much higher
in a b18c1 its a 81mm. i think you can go as far 87mm when sleeved to the max size.
you'll have to buy a whole new bottom end and send your block off to a machine shop to re-sleeve it.

i am going through this myself with my GS-R/CTR motor!!! not sure if its worth it on a D but go for it.