Symbolic variable. Some numeric or string quantities may have to be represented by a symbolic variable rather than an explicit value. SYMBOL is an experimental approach towards this and the details have not yet been worked out.
Hypothetical... Suppose two objects are contrained to have numerical values
which sum to a constant value (e.g. the occupancies of two or more atoms must
sum to 1.0). We might write:
<XVAR TYPE="symbol" NAME="pos1">C13occ</XVAR>
<XVAR TYPE="symbol" NAME="pos2">1-C13occ</XVAR>
Yet to be worked out... At present any variable.