Mechanic
Protocol: yu-gi-ohXyz Summon
Official Definition
A Special Summon using two or more monsters of the same level, which become overlay units (materials) for the Xyz Monster.
Technical Rulings
Xyz Monsters rank is not a level. They cannot be used for Synchro or Tribute Summons based on level.
Execution Examples
"Stacking two Level 4 monsters to Xyz Summon a Rank 4 monster."