If you understand MCNP5 theory, you understand 90% of MCNP6 theory.
Allowing a particle to survive an interaction with a reduced weight rather than being terminated, thereby increasing the information gathered per history. Geometry and Criticality mcnp5 theory manual
Common topics include: thermal scattering law implementations, tallies in repeated structures, or weight window generation. Refer to the manual’s index – it is surprisingly thorough. If you understand MCNP5 theory, you understand 90%