A more thorough explanation:

Term: in pari causa
Definition: In a situation where two parties have equal rights or are equally affected, the person who possesses the property or right usually wins. For example, if two people claim ownership of the same car but one has possession of it, the possessor will usually win the case because they have a stronger claim in pari causa.
Example: Two neighbors both claim ownership of a tree that is on the boundary between their properties. However, one neighbor has been taking care of the tree for years and has even built a treehouse in it. In this case, the neighbor who has possession of the tree will likely win the case because they have a stronger claim in pari causa.