Although the various default unification operators differ in their particulars, most involve something like Carpenter (1992)'s credulous default unification as one step.
the result of credulously adding the default information in G to the strict information in F is: cred-unify (F, G) is the result of unifying F with the maximal consistent subset(s) of the atomic constraints in G. A subset of constraints is maximally consistent with respect to the subsumption ordering if no constraint can be added to it without creating an inconsistency.