I might suggest a citation index to accompany the site_index value so users outside the interface could see what the site index means. sadly, we would have to use a number constant. I've got libs that expose all kinds of site index values and an index is used to interface with multiple species presented in a single paper (like hann and scrivani 1987 or example). For example:
double ht_from_site_index( 23,
or whatever these are supposed to be...
unsigned short sp_code; /* standard species code */
double site_index; /* site index */
/* MODXXX */
unsigned short site_index_func;
unsigned short site_index_sp;
double max_sdi; /* maximum SDI for the species */
double tolerance; /* some models use a tolerance index */
is is this overboard.....