can periodic group may be define as a super descriptor?
This is from the documentation Wolfgang noted above:
"A superdescriptor is a descriptor created from several fields, portions of fields, or a combination thereof. "
A Periodic Group is NOT a field. It is an “abbreviation” for a bunch of fields. As noted in the documentation, fields within a Periodic Group are candidates for a super descriptor; the periodic group itself is not a candidate for a super descriptor. Carrying this one step further, a Periodic Group cannot be a descriptor field (because it is not a field), although fields within a Periodic Group can be descriptors.
The same is true for “Group Names” in Natural. They have neither format, nor length. They are NOT fields.