2D doesn’t always equal UID, but UID always equals 2D.
It almost sounds like a brain-teaser or a riddle, doesn't it? If this statement sounds confusing to you – don’t worry you are not alone. Understanding the relationship between the 2D bar code and UID is an important step in becoming more comfortable with UID as a whole. A two dimensional (2D) Data Matrix bar code is simply another bar code symbology just like Code 39 or Code 128, etc. Companies may choose to use this symbology when they have large amounts of data they want to incorporate into the bar code or they have a small space in which to put the mark. Consequently, a 2D bar code doesn’t always equal a UID.
On the other hand, the DoD Guide to Uniquely Identifying Items specifies the use of the 2D bar code as their symbology of choice regardless of the information contained within the bar code – so the 2D bar code essentially is the UID. In other words, you can't have a UID without a 2D bar code.
So the next time you see a 2D bar code don't assume it is for a UID application. There are other non-UID related reasons for using this unique bar code symbology.