Primitive Engine (PE) is one of the PICA200's four vertex processor units and provides some unique features which are used to implement a geometry shader stage and variable-size primitive rendering.
The full functionality of PE is not yet understood and remains to be reverse-engineered.
Variable-size primitives are implemented by prefixing each per-primitive sequence of indices in an index array with a primitive size. This is used for various effects, for example Catmull-Clark subdivision and Loop subdivision. It is unknown how this feature is enabled specifically.