[mkgmap-dev] Garmin feature codes

I am trying to understand the mysteries of Garmin feature codes. I am guided by the list that is in the cGPSmapper manual plus other sources, some of which have quite obscure codes, in creating my personal .typ file that is used in conjunction with mkgmap. Such codes are necessary in order to obtain greater variation in representation of, say, various land use features. Yet I cannot ascertain whether these codes are recognised by my Legend HCx until I upload the resultant gmapsupp.img, and often they are not recognised. Does this depend upon the individual GPS device or its firmware? Does anyone know of custom codes, particularly for polygons and polylines, that are usually recognised by Garmin devices? Bob Hawkins