These three resources must be located between special marker lumps so that Doom knows what it is looking at. Other than defining the beginning and end of a graphics section, these lumps contain no data and are 0 bytes long.
The markers consist of names X_START and X_END, where X is the first 1 or 2 letters of the appropriate resource. For example, sprites should be located between S_START and S_END markers. SS_START and SS_END are usually used for user WAD files. Similarly Flats are F_START and F_END (again FF_START and FF_END in user WAD files). Patches are not required to have any markers. Some [[lump]] management utitilies utilities require P_START and P_END.
===Miscellaneous===