Feedback is a GL render mode. The mode is selected by calling ggllRReennddeerrMMooddee with GGLL__FFEEEEDDBBAACCKK. When the GL is in feedback mode, no pixels are produced by rasterization. Instead, information about primitives that would have been rasterized is fed back to the application using the GL. See the ggllFFeeeeddbbaacckkBBuuffffeerr reference page for a description of the feedback buffer and the values in it.
ggllPPaassssTThhrroouugghh inserts a user-defined marker in the feedback buffer when it is executed in feedback mode. _t_o_k_e_n is returned as if it were a primitive; it is indicated with its own unique identifying value: GGLL__PPAASSSS__TTHHRROOUUGGHH__TTOOKKEENN. The order of ggllPPaassssTThhrroouugghh commands with respect to the specification of graphics primitives is maintained.