The concept of numbered sequencing is used in a few places in the admin, mainly with the Products. This help page will explain the concept of numbered sequencing.
The sequence number is just an arbitrary number that tells the program what order to display the items in, and enables you to order the items as you wish by assigning them numbers from lowest to highest.
It is best to give items sequence numbers that are incremented by 100 or 1000. This is so that it will be easier to reorganize them later if need be. For example, if you had 5 items and gave them sequence numbers of 1, 2, 3, 4, 5, then what happens if later on you want to add a sixth item in between the others? You would have to modify several existing items\'s sequence numbers to make space for the new item in between; this would be a pain. However, if you gave your five original items sequence numbers of 100, 200, 300, 400, 500, then it\'s easy to add a sixth item in between by simply using a sequence number of 350.
Also, note that sequence numbers are only relevant to "neighboring" items - items that are on the same level, with the same parent item. In other words, sequence numbers only matter for items that will be shown in the same list with each other.