Sit makes it possible to use slabs and stairs like chairs. Simply rightclick the top side of a right side up slab or stair block with an empty hand to sit on it. To exit, press the key you use to exit a minecart (by default SHIFT). Half beds (a bed where only one half is present) work as well. Usually, slabs/stairs from other mods are supported, however if any do not work, please contact me.
There's a single configuration option: block_reach_distance. It defines how many blocks a player can be away from a slab/stair to still be able to sit on it. This is capped by the player's block reach distance.
You are allowed to use this mod in any modpack.