Chemical formula. The primary purpose of this is to say what the molecule is, not to represent ideas about it. No present method covers all molecules, and for many we have only partial info (e.g. stoichiometry). FORMULA allows for one connexion table in the content - but more than one FORMULA is allowed within MOL to cover multiple components (especially in crystallographic files).
The primary use for the generic content (ARRAY/XLIST/ARRAY) are connexion tables. The connexion tables can be textual (e.g. SMILES) or the components of an atom-bond based table, following the same convention as in ATOMS and BONDS. In FORMULA both atom and bond arrays can be used, which will normally be of different sizes. XVAR can also be used for reference numbers, etc (MEDLINE, SWISSPROT, Cambridge, etc)
FORMULA supports CONVENTION/DICTNAME so that differing conventions can be used.
The following BUILTINs are provided (See mol-var-bui.ent).