A concrete definition. This is generally used when you want to define a prim for the very first time, or when a prim needs to be redefined. defs always contribute to scene composition.

def Xform "MyTransform"
{
  # ...
}

def can also be type-less and is usually used in context of composition, which will fill in the actual type.

def "MyPrim" {
  # ...
}

This tells USD that a prim is to be overridden. If the composition engine cannot find the prim targeted by the over specification, the override will be ignored and will not contribute to scene composition.

over "SomethingThatAlreadyExists"
{
  # ...
}

An abstract prim definition that does not contribute to stage traversal by default. Meaning, it does not show up in the stage hierarchy. Other prims can however, inherit from this class (more on that in the next chapters)

class "_MyClass"
{
  # ...
}