When you sign up to BOMA you'll see there are two options - monthly or annual.
Signing up to BOMA on a monthly plan gives you the most flexibility. You aren't locked into a contract and can cancel your account at any time.
If you start out on a monthly plan you can upgrade it at any time to an annual plan.
Signing up to BOMA on an annual plan offers less flexibility as you are committed for one year. However you save 15% vs the monthly plan.