This rule is added by default in 1.2 f/w to support ad hoc 1-stage gateway calls (using OBiTALK trunk only at the moment). You can configure the obi number of another OBi (gateway) in a speed dial (1-99). To call out using that gateway, dial <speed-dial> * <target-number>. Since AuthUserID/AuthPassword are not specifiable in a speed dial, this method only works if the gateway does not require HTTP Digest authentication.
x? means 0 or 1 x, so [1-9]x? maps to 1-99, a speed dial.