There’s a heap of documentation for D-Phones here:
https://wiki.asterisk.org/wiki/display/DIGIUM/Digium+D-Series+Phones
And for A-Phone here:
https://wiki.asterisk.org/wiki/display/DIGIUM/Digium+A-Series+Phones+for+Asterisk
D-Phones do many interesting things that A-Phones don’t. The wiki docs will give you ideas. A-Phones do a few things that D-Phones don’t.
D-Phones run native JS apps that you can load onto the phones, written to this API:
https://wiki.asterisk.org/wiki/display/DIGIUM/Phone+API+Documentation