I have had a Digitrax Big Boy for many years I recently bought a steam loco equipped with a Soundtraxx decoder I have been able to utilize function keys 0-4 but there is quite a few more that the decoder is capable that I don't know how to access example f6-9
Production of Big Boy was from 1994 to 1998.
It was superseded by other starter sets and command stations.
Big Boy can access Functions 1, 2, 3 and 4, but no higher.
To reach the higher function numbers will require upgrading to a more current command station and throttle combiniation.
The Big Boy can still be used as a booster.
From page 6 of the documentation:
Connect the two GRAY terminals of the DB100 with a piece of wire to force the DB100 to power up as a booster.