You can review the inclusions and limitations here.
The most common reason to upgrade is to remove the limit of 27 STP submissions. Each one of the entities inside your free software will have 27 free submissions and, when that limit is exceeded, you can pay to upgrade them as each one reaches their submission limit.
For example, if you are submitting STP for these three different entities:
Client 1 (weekly payroll, weekly STP submission): software upgrade will be required after 27 weeks
Client 2 (monthly payroll, monthly STP submission): software upgrade will be required after 27 months
Client 3 (quarterly payroll, quarterly STP submission): software upgrade will be required after 6+ years
At their relevant time to upgrade, you will need to pay $2 per employee (minimum charge of $10/month) for each entity you are reporting for.
Please review our STP upgrade page for more information and the benefits of upgrading.