For nCloth, self collisions occur between the nCloth objects’s components, such as its vertices or faces. Self collisions are necessary for animated nCloth to simulate the interactions of cloth when it comes into contact with itself. For example, when an nCloth curtain panel is being pushed open along a curtain rod, it begins to sway and pleat against itself, causing the cloth to deform. You can also imagine how a flag might fold in on itself when the wind dies down, or how the full skirt on a dancer collides when the dancer changes direction.
For nParticles, self collisions occur between the individual particles that belong to same nParticle object. Like collisions between nParticles and other Nucleus objects, self collisions involve an exchange or transfer of energy. nParticle self collisions can be used to approximate interactions between rigid objects. For example you can instance geometry with nParticles and have the geometry collided with each other.
For nHair, self collision occur between a hair system's dynamic curves.
For self collisions to take place, the Self Collide attribute must be turned on in the Collisions section of the nClothShape or nParticle Attribute Editor tab. In addition, you can set which of your nCloth’s components (Vertex, VertexEdge, VertexFace, Full Surface) participate in its self collisions with the Self Collision Flag drop-down list in the Collisions section of the nClothShape tab. For nHair, you can set the Self Collision Flag to either Vertex or Edge. You can also increase and decrease the quality of your nCloth and nParticle object’s self collisions with the Max Self Collide Iterations and Self Collide Width Scale attributes.