The above are compiled into the therion executable in some way so that therion knows about those area symbols. If I want to override one of the above area symbols with my own metapost code I can do that by including the metapost code in thconfig.
Yes, you can override only existing symbols.
But if the area symbol is not in the list above I can't just create metapost code for say "a_something_ASF" as therion will not know about it even if it's in thconfig.
Is that right?
Well, symbol types/subtypes are in fact defined in thpoint/thline/tharea.h/.cxx files. In metapost files, there is only their visualization defined, therefore it does not help to define only new metapost symbols.
PS. It does not worry me if there is no option in xtherion GUI for selecting say an area 'flowstone' if I have an a_flowstone_ASF, as I can select say clay for an area and then edit the th2 file file with vim and change clay to flowstone.
Soon we would like to add some generic point/line/area symbols, that could be used in cases, when desired symbol does not exists in therion. Regards, S.