A .ai can keep its variables and data sets in a variable library in
its editing data. VectorCraft doesn't read that library, and such a
file opened with an empty Variables panel and no warning about it.
The reader now counts the library's variables and data sets in the
editing data's /Document dictionary, and opening the file adds a
warning: "the file's variables and data sets weren't read (2
variables, 3 data sets): the Variables panel doesn't list them". The
same warning appears when the layers can't be read and the file opens
from its PDF part. A PDF that opens from the editing data it carries
gets the warning too. A library with no variables and no data sets
gives no warning.
Docs: docs/target-app-parity.md.