As Helmut Nisters says, forms and varieties (like cotypes) belong to
yesteryear. The rule is that forms and varieties are to be treated as
subspecies where possible. One case where it is not possible is this:

Nisteroconchus helmutius, variety innsbrueckensis, form musei.

In this case, we have genus, species, variety (= subspecies), and form (=
lower in rank than subspecies, and therefore ignored by the Code). In
Latin, varietas and forma.

Anyone can still name and use forms below the subspecies level if they
like. They are simply not valid under the Code, nor are they protected by
its rules. They are ignored by it. (And as usual, the rules for plants are
different.)

Andrew K. Rindsberg
Geological Survey of Alabama