pUo is a symmetrical relation, i.e., pUo implies oUp. That they belong to a unit is true for p and o in the same way, though their roles in the unit may be different (for instance, if U is a causal unit). However, pLo is non-symmetrical since it does not imply oLp
Why does pUo imply oUp, but it pLo does not imply oLp?