It is a catcher to route inbound analogue calls. If you have an analogue card plugged into your box then this trunk will be used to route the inbound calls. It really should be called tdm-in because it will also catch digital calls on circuits which don't have DNID presentation enabled.
It has alias contexts of [from-pstn] for compatibility with dahdi_genconf and [fxo] and [digital] for the Warp.
Kind Regards
S