Help with dahdi trunks in freepbx 2.8.0

Hi !

I upgrade freepbx from 2.7.x to 2.8.0.x, now I can see in trunks basic module that we can create some dahdi trunks.
I actually use zap trunks for digium cards.

Configure dahdi trunks look like zap trunks configuration but I don’t understand what ‘DAHDI Trunks Group 0 ascending’ and ‘DAHDI Trunks Group 0 descending’ means in Outgoing settings.

How to configure this parameter ? What’s the difference between them ?
Thanks in advance for help