This line shows accession number of the entry data.
A unique accession number is issued to the data submitter by each of the three data banks. The accession number is composed of 1 alphabet character and 5 digits (ex. A12345) or 2 alphabet characters and 6 digits (ex. AB123456). The former style was used in 1980s, but later the latter style was introduced because of data explosion.
The alphabet part is called "prefix". Please refer the prefix list.
If multiple entries are united to an entry, or if an entry is extensively modified after the submission, the responsible data banks may assign a new accession number to it. In these cases, the new accession number is called the primary accession number, and the old accession number(s) is/are called the secondary accession number(s). In the flat file, the primary accession number is indicated first, then the secondary accession number(s) follows. You can find the same updated entry with both the primary and the secondary accession numbers.
example ACCESSION AB999999 AB888888 AB777777
The set_version goes up for every update of the dataset.
example ACCESSION ZZZZ01000001 ZZZZ01000000
This (ACEESSION) line shows a number assigned by INSDC to a resource. The number is composed of 5 alphabet characters and 7 digits (ex. ZZZZZ0000000).
An accession number assigned to an entry of a resource units is displayed in the MGA lines.
example ACCESSION ZZZZZ0000000