This is short for "tier of service." Memberships at IAA are divided into tiers, with each tier containing all the benefits of the tiers below it while adding new ones.

"Benefits" means:
What types of actions you can perform,
What types of information you can access.

The current tiers are:
Guest (T1, free, may require guest login)
Observer (T2, free, login required)
Active Member (T3, $10/month, login required)
Interactive Member (T4, $20/month, login required)

For a complete chart of all tiers and benefits go here.
You can subscribe to one of the paid tiers at Stanley Jordan's Pateron site.

» IAA Glossary