> This is the outcome of querying RDB$MECHANISM.
> How do I know which one means FREE_IT?

Negative one. FREE_IT is recognized by sign.

> SELECT RDB$MECHANISM,COUNT(*)
> FROM RDB$FUNCTION_ARGUMENTS
> GROUP BY RDB$MECHANISM
>
> RDB$MECHANISM COUNT
> ============= =====
>>> -1 3
> 0 20
> 1 56
> 3 2
> 5 5

SY, SD.