The 5-4-2 billing form of an NDC, produced by padding a leading zero into whichever 10-digit segment is short.
HIPAA electronic transactions define a fixed-width 11-digit NDC field, so every 10-digit FDA code has to be normalised before it is transmitted. The padding rule depends on the source format: 4-4-2 pads the first segment, 5-3-2 the second, 5-4-1 the third.
The zero is inserted, never appended. 0002-7510-01 becomes 00002751001, not 00027510010. Getting this wrong produces a syntactically valid code that points at a different product or at nothing.