For ASL_FileRequest and ASL_FontRequest, popasl class offers a a standard start/stop handling. When a file requester is opened, MUI splits the string gadgets contents into a path and a file name and uses these as initial paremeters for the requester. Font popups translate a font into a name/size pair, e.g. ''topaz/8''. You can override these translations by specifying a MUIA_Popasl_StartHook and a MUIA_Popasl_StopHook.
For ASL_ScreenModeRequest, no standard handling is available. Using such a popup without Start and Stop hooks won't make much sense.