The UID is a totally unique identifier containing data elements which are encoded into Data Matrix bar codes that are applied to every qualifying government item.
MIL STD 130 requires that all qualifying end item deliverables, government furnished property in possession of contractors (PIPC), and legacy items must be marked with a machine readable 2D Data Matrix bar code.
Data Matrix (sometimes spelled datamatrix) is a two-dimensional bar code, which can store as many as 50 data characters and the additional high-level control characters required for item marking.