Hi Gordon
Yes - you have answered it.
In V2/V3 trunks and DiDs were quite blurred. You could define a trunk and assign a DiD to it in one entry. However, while this is OK for people who know how it works, it can be confusing, particularly for new users and those coming from FreePBX type systems where a fairly strict distinction is made between a trunk and a DiD. So we split them in V4. Trunks can no longer double as DiDs in V4 (well, they still can but only within the limits of a well-formed DDI i.e. a string of digits). However, in inbound routing, you can now assign any valid Asterisk dialplan to a DiD so it can quite happily be a number group (i.e. _6744XX.) and it can have any valid asterisk dialplan character included in it. It's more intuitive because it nicely falls into the routing section (along with the older "Routes" for outbound).
Best
S