This property is the main rule in determining the root part of an assembly. It is an integer between -127 and 127 that takes precedence over all other rules for root part sort (including the weird rules based on part
BasePart/Size|size). A part with a higher RootPriority will take priority over other
BasePart/Anchored|unanchored parts with equal
BasePart/Massless|Massless values and a lower RootPriority.
Use this to control which part of an assembly is the root part and keep the root part stable if size changes.
articles/understanding root parts|Understanding Root Parts, an article documenting what root parts are and how to use them