CollisionGroup

Default layer, always collides with everything.

Conditionally solid means that the collision response will be zero or as defined in the table when there are matching interactions

Collides with nothing but world, static stuff and triggers

Things that are dissolving are in this group

Blocks entities not permitted to get near moving doors

First unreserved collision layer index.

for any entity inside a vehicle

Collides with everything except interactive debris or debris

Collides with everything except other interactive debris or debris

Generic NPC group

Used so NPCs in scripts ignore the player.

Used for NPCs in scripts that should not collide with each other

This is how you turn off all collisions for an object - move it to this group

Doors that the player shouldn't collide with

For HL2, same as Collision_Group_Player, for TF2, this filters out other players and CBaseObjects

Projectiles!

Nonsolid on client and server, pushaway in player code

Trigger layer, never collides with anything, only triggers/interacts. Use when querying for interaction layers.

vehicle clip brush to restrict vehicle movement

for any weapons that need collision detection