You may use a miscellaneous prefix: "MISC".  Or something that fits into your system.  

Product IDs could be important, depending on the industry.  You may consider a more elaborate system.  In my old retail job, product IDs were never auto-generated by the machine, but determined manually by my boss and her production staff, in a very systematic way.

If you are a wholesaler, your product IDs could be seen by the public -- on hangtags, product listing given to buyers, etc.  And you don't want people to see numbers like 69, 911, 666, etc.  You get my meaning.

Hey Guys, I have figured out the code for Generating a "Custom ID Code" for 'Estimate, Contracts, or Invoices". But the VBA Code keep create ID Numbers that are in the Middle where Deleted "Autonumbers" were, not the "Last Item Number". Here what I got:

Function GenerateEstimateVerNum() As String

    Dim customNumber As String
    Dim lastNumber As String
    Dim frontPrefix As String
    Dim backPrefix As String
    Dim verCount As Long
    
    frontPrefix = "Rev."
    backPrefix = ".V"
      
    lastNumber = Nz(DMax("EstimateVersionNumber", "EstimateT"), 0)
    verCount = DCount("EstimateVersionNumber", "EstimateT")
    
    customNumber = frontPrefix & Format(verCount + 1, "0.0") & " " & backPrefix
    GenerateEstimateVerNum = customNumber

End Function