When you look at a movie on the Movie Store, there are two things to look for. One is the UV logo on the left. This means that the movie is UltraViolet enabled, and if you purchase it from the Movie Store, it will be checked into your UV locker (and conversely, if you check it into your UV locker, you will be able to download it).
The other thing to look for, just above the name of the studio, is the "Disc to digital" line, which will tell you if that movie is eligible for a Disc-to-Digital transaction.
These are similar concepts, but not identical.
If Disc-to-Digital is available for a movie, it's typically $1.99 to download the same resolution movie (e.g. Blu-ray quality for a Blu-ray disc). But, if the disc comes with an UltraViolet code in the package that you can check into your UV locker, the download is at no charge.
You may find that movies that come with the UV code in the package are sometimes not enabled for Disc-to-Digital, because the studio has already provided a means to get the digital copy.