There are 3 stages (meaning levels) of IS-BAH certification.
When you are just starting with IS-BAH, you are starting with Stage 1. You cannot skip Stage 1.
After you received your Stage 1 certification, you hold it for 2 years. After that, you have to advance to Stage 2.
You can then stay with Stage 2 forever, having Stage 2 audits in a rythm of 2 years to confirm your status.
Stage 3 is for the organizations that want to go the extra mile. With Stage 3, you commit yourself to complying with advanced SMS methods.
Stage 3 can be a competitive advantage for FBOs at airports with multiple IS-BAH certified FBOs.