9.6 years ago
To add to @Lyco's answer: they are not simply assigning 'new' accession number to old entries. That only happens when items are merged or split, and the original numbers are indeed kept as secondary accession numbers.
Entries can have more than one accession number. This can be due to two distinct mechanisms:
a) When two or more entries are merged, the accession numbers from all entries are kept. The first accession number is referred to as the ‘Primary (citable) accession number’, while the others are referred to as ‘Secondary accession numbers’. These are listed in alphanumerical order.
b) If an existing entry is split into two or more entries (‘demerged’), new ‘primary’ accession numbers are attributed to all the split entries while all original accession numbers are retained as ‘secondary’ accession numbers.
Also be aware that you: "should always use the primary accession number of an entry in any citation and link since it is the only unique stable identifier for an entry."
(Ohh and the P actually meant "protein" but then they ran out of P's)