autostep_sbw controls the maximum difference in height at which a player will "step" onto an above water sector (lotag 1) or floor aligned sprite (cstat] 32) (see spritebridge) without needing to jump. The default value is 1024.
Making the value smaller allows the player to climb higher sectors and wall aligned sprites, while a larger value means the player will not be able to climb over smaller and smaller heights in sectors and sprites.
If you are making a permanent change to this value, it is best to do it from EVENT_ENTERLEVEL so the game never overwrites your value with the default.
For normal sectors and view/wall aligned sprites please see autostep.