The STA is a registry of tartans and provides services to its members and the public to help sort out which tartans are which and to maintain a history of where they came from. From their own pages:

We are a registered Scottish Charity with the major objective of promoting a deeper knowledge of Scottish Tartans, their origins, manufacture, use, history and development and we study, record and stimulate research on the subject.

A vitally important objective has also been to compile and maintain the International Tartan Index to record and document all known historical tartans and to provide a free, dependable and accountable information resource for the public and a 'register' for the Recording of all new tartans.
In no way do they grant or restrict wearing whatever tartans or polkadots in anyone's kilts nor do they proscribe how a kilt should be constructed.

Regards,
Rex.