cht currently recognizes the element symbols C, H, O, N, P, S, Si, B, Br, Cl, F, I, Al, As, Ba, Be, Bi, Ca, Cd, Co, Cr, Cs, Cu, Fe, Ga, Ge, In, K, Li, Mg, Mn, Na, Ni, Pb, Rb, Sb, Sc, Se, Sn, Sr, Te, Ti, Tl, V, Zn (that is, all main group elements except the noble gases, and the first row of transition metals) and the abbreviations Ac, Ade, Bn, Bu, iBu, tBu, Bz, BOC, Cyt, CE, DBAM, DMAM, DMTr, Et, Gua, Me, Ms, MOC, MOM, MMTr, Ph, Pr, iPr, Tf, Thy, Tol, Tr, Ts, TBDMS, TBDPS, TMS, TMTr, Ura, Z. It can handle two levels of parentheses; e.g. P[OCH(CH_3)_2]_3. When cht detects any duplicate (overlapping) bonds in the drawing file, it will prepend an exclamation mark to the calculated (and likely wrong) sum formula.
A label that is offset from the corresponding bond end by more than five pixels is not recognized, so that the default CH2 or CH3 group enters the calculation in its place.
In general, one should check the plausibility of the generated sum formula wherever possible.