[…] let us extend the notion "θ-marking" in the following way. We will say that α "θ-marks the category" β if α θ-marks the position occupied by β or a trace of β. Note that α subcategorizes a position but θ-marks both a position and a category [...] We can now assume that without exception, if α subcategorizes the position β, then α θ-marks β and θ-marks a category "C" such that "C" or a trace of "C" occupies the position β.
- Chomsky (1993), Pag. 37

[…] we may interpret the notion θ-marking in the following way: if a structural position that can be θ-marked is obligatory, then it is obligatory θ-marked by an element that may θ-mark it; if such a position is only optionally present, then θ-marking of this position is correspondingly optional, and will apply just so as to satisfy the θ-criterion.
- Chomsky (1993), Pag. 40