Well, I think there's no official rule when to use DAL1 and when to use DAL2. In my opinion there's only "DAL": a place where I can link code to hooks on the table and/or field level.
Here's my interpretation:
There's been some evolution in DAL programming. There has been an initial set of hooks and functionality. This has been extended. The old version is nowadays called DAL1, the latest version is called DAL2 (or "Extended DAL"). Basically Dal1 is focused on table level hooks and Dal2 introduced (added) hooks on field level.
There's still a script type "Dal" (Dal1) available, but I think it is for backward compatibility only. When I'm creating new dal scripts, they are always of Dal2 type.