Stimulated-emission-depletion direct laser writing allows for performing 3D optical lithography beyond the Abbe diffraction limit. However, the underlying mechanisms and limitations are poorly understood. In order to clarify for the case of 7-diethylamino-3-thenoylcoumarin (DETC) as photoinitiator in pentaerythritol triacrylate, transient photoluminescence experiments as well as lithography experiments with variable repetition rate are performed. In addition, several coinitiators and DETC derivatives are investigated. While the photoluminescence of the DETC photoresist exhibits a two-photon excitation behavior and can be largely depleted, lithography under the same conditions surprisingly shows an effective three-photon excitation behavior at high repetition rates and an effective four-photon behavior at low repetition rates. From the comprehensive investigation, it is concluded that at high repetition rates, residual absorption of the depletion laser is the limiting mechanism, whereas at low repetition rates an effective four-photon process leads to direct radical generation that cannot be depleted.