UPC-E is a compressed variation of the UPC-A symbol that is used where physical label space is limited. Compression works by squeezing out zeroes when printing the barcode, and then re-inserting them automatically in the barcode scanner. It is important to understand that every UPC-E code can be de-compressed into its UPC-A equivalent, but not every UPC-A code can be compressed to UPC-E.
Since compression works by squeezing out zeroes, the original UPC-A code needs to have some spare zeroes to begin with. Only manufacturer GS1 ID numbers beginning with zero are eligible (Number System 0). In addition, there are four rules that determine what UPC codes can be printed using the compressed UPC-E format: