[…] 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