Each record has a unique reference number associated with it, by type, called the Record Id as explained in Record Types and Record Id.
Additionally, on the All tab (often the Main tab) of the Property Box for most record types there is a Custom Id.
Users who like to have their own identifiers, can use this Custom Id field to associate an Individual, a Family, a Note, a Source, a Repository, or Media record with their identifier. On the All tab, right-click the record Name and use Add Miscellaneous > Add Custom Id or for some records just Add Custom Id.
If necessary, the Custom Id can be customised into the Main tab via the Menu > Customize Data Entry option, so it can be viewed and edited directly.
Also the Custom Id can be customised into a Records Window column via Lists > Configure Record Window Columns, and sorted into order. The default Expression is %INDI.REFN% which will sort alpha-numerically. To sort purely numeric Custom Id then Edit the column Expression to use =TextToNumber(%INDI.REFN%). Similar Expressions can be used in Diagrams, Reports, Queries, etc.
There is a method of renumbering the Record Id with the Custom Id using Tools > Record Identifiers, but generally the Record Id should be left alone.
Every record that allows a Custom Id actually allows an unlimited quantity of them such as %INDI.REFN% and %INDI.REFN%. Also each Custom Id offers a subsidiary classification Type field such as %INDI.REFN.TYPE%. As described above, all these fields can be customised into the Property Box, Records Window, etc, and the Type field tested to conditionally include classes of Custom Id, e.g. =TextIf(%INDI.REFN.TYPE%="Prime",%INDI.REFN%,"").