:WP:NOTMANUAL.  There's no reason to try to fully document the format.  Thomson-Reuters established and maintains the specification, and so the Reference Manager website should probably be the authoritative reference, but note that (i) they do change the spec and (ii) you're looking at an out-of-date version.  The spec is now hosted at , and this describes the C[1-8] custom fields.  There appears to be some precedence of trying to use the "custom" fields in a semi-common way (for journal articles, T-R uses C2 for PMCID, for example) & for reserving the U[1-?] fields for "user-defined" way that programs don't try to enforce.  But, in any case, I would not make a radical change based on out-of-date info.