BrickLink assigns a sequential number for all items in the Sets item type. The sequential number increases for each additional catalog entry using the same set number. Sequential numbers might be used for a variety of reasons, six of which are described below.
For Reasons of Commerce
An example is Set 71001-1. This set includes a random figure, but BrickLink reused the set number to refer to an artificial grouping of all the random figures that could have been included (17 figures, in this case). The grouping was given the sequential number 71001-2.
For Set Reissues
An example is Set 4962-1. This set was released in 2005 as a Duplo LEGO Ville set. Nine years later, in 2014, the set was reissued with essentially the same contents and title, but in different packaging without the LEGO Ville identifier. The reissued set has the sequential number 4962-2.
An example is Set 41353-1. This advent calendar includes 24 subsets numbered 41353-2 through 41353-25.
Packaging Type Differs
An example is Set 7223-1. This set comes packaged in a polybag, but a promotional version was released packaged in a cardboard box. That set has the sequential number 7223-2.
Set Contents Differ
An example is Set 7075-1. This set comes without a motor, but a limited edition includes a motor for the ship and has the sequential number 7075-2.
The LEGO Group Reuses a Set Number
An example is Set 6648-1. The number 6648 in this example is the set number and the number 1 after the hyphen is BrickLink's sequential number. The LEGO Group reused this set number for a completely different set, so there is also a catalog entry for 6648-2. BrickLink does not identify releases in any particular order, including chronological.