Sorting may be handled differently or similarly in different features so any documentation would need to be specific to the feature.
The way it works in the List/Links feature is that it sorts by the ViewOrder field (which corresponds to the Sort Rank label) and then by the Title of the link. By default the ViewOrder is 500 and if never modified that leads to links being sorted alphabetically since ViewOrder is the same in every row. So an item can be moved down the list by a lower sort rank or up the list by a higher one. The field itself is of data type int so it has whatever limits go along with sql int data type.