What is the logic behind the delivered default enrollment options?

What is the logic behind the delivered default enrollment options below?

• New – Default; Current – Same Enrollment and Rates
• New – Default; Current – Same Enrollment but Default Rate

We've confirmed that these options process correctly during passive enrollment. However, when we create a custom formula, it doesn't seem to trigger during passive enrollment. We're not sure whether the issue lies with our formula, so we're hoping to understand the underlying logic of the delivered default enrollment options to help us troubleshoot.