/chill puts snow layer on top of slabs and other blocks it can not normally fall on.

[Anna Bakker]

When using /chill over a area, it will put snow layer on slabs (both top half and bottom half) and steps,

When snow layers on slabs are updated, they will break.

Normally when natural snow weather occurs, it will not place snow on top of bottom slabs (however it will place it on double slabs) or steps. Snow golems also can't place snow layer on steps or slabs, and players can not place the snow layer block on steps or slabs normally.

If someone uses /chill on a build with steps or slabs on the roof, it makes it look like the snow layer is floating above the slab/step.

[Riot]

The chill command's script is just a thin wrapper around worldedit's freeze or equivalent command with a constrained radius and a cooldown (same with fixwater, lava, grassify, mycelify and heatwave) so please post any bugs with the world modification process itself on WorldEdit.