Collision filtering

Two shapes collide when each one's Categories appears in the other's CollidesWith. The test is symmetric, so making one side ignore the other is enough to disable the pair.

[Flags]
enum Layers : ulong
{
    World  = 1 << 0,
    Player = 1 << 1,
    Enemy  = 1 << 2,
    Debris = 1 << 3,
}

var playerFilter = new CollisionFilter
{
    Categories = (ulong)Layers.Player,
    CollidesWith = (ulong)(Layers.World | Layers.Enemy),
};

body.AddCapsule(capsule, ShapeDefinition.Default with { Filter = playerFilter });

A shape usually belongs to one category and collides with several. CollisionFilter.Default belongs to every category and collides with everything, which is what a shape gets if you say nothing.

The same filter applies to queries, not only to shape-versus-shape collision, so debris that ignores the player is also invisible to a ray cast filtered to the player's categories.

Groups, for pairs and ragdolls

A non-zero Group overrides the masks entirely:

Group	Effect
negative	Shapes sharing it never collide
positive	Shapes sharing it always collide
zero	No effect; the masks decide

// Every limb of one ragdoll ignores every other limb of the same ragdoll.
var filter = CollisionFilter.Default with { Group = -ragdollIndex };

This is the cheap way to say "these objects never collide with each other" without spending a category bit on them, and it scales: each ragdoll gets its own negative number.

One pair, without spending anything

For a single pair that must not collide — a projectile and the turret that fired it — a filter joint is more direct than either mechanism:

world.CreateFilterJoint(FilterJointDefinition.Between(turret, shell));

As a side effect the two bodies stay in the same simulation island, so they sleep and wake together.

Changing a filter later

shape.SetFilter(newFilter);                          // recomputes contacts
shape.SetFilter(newFilter, recomputeContacts: false);

Recomputing is what makes a change take effect on contacts that already exist. Skip it only when you are about to move the shape anyway.

Choosing between the three

You want	Use
Layers: players, enemies, world, debris	Categories and CollidesWith
A set of objects that ignore each other	Negative Group
Exactly two bodies that ignore each other	FilterJoint
A shape that detects but never pushes	Sensor