Indicative sentential subjects obligatorily have complementizers while infinitival sentential subjects may or may not have a complementizer. Also if is possible as the complementizer of an object clause but not as the complementizer of a sentential subject.
If the sentential subject is finite and a complementizer does not adjoin in, the assign-comp feature of the S0 node of the embedding clause and the root node of the embedded clause will fail to unify. If a complementizer adjoins in, there will be no feature-mismatch because the root of the complementizer tree is not specified for the assign-comp feature. The comp feature nil is split into two assign-comp features ind_nil and inf_nil to capture the fact that there are certain configurations in which it is acceptable for an infinitival clause to lack a complementizer but not acceptable for an indicative clause to lack a complementizer.