I will do my best to make some since. 
I have a form for entering data into a table named dispatchdetatils  The 
Combo Box named vehiclebox on the form shows vehicle license plates numbers. 
When a vehicle is chosen a new record is started and the new record milesout 
field on the dispatchdetails table is populated from the max of the milesin 
field. Below is what I have in the After update of the combox. 
What I would like to happen also is if there is a milesin field that is null 
then do not start the new record but have a pop up appear stating the vehicle 
is already out. Thank you in advance

Private Sub VEHICLEBOX_AfterUpdate()
       If Not IsNull(Me.VEHICLEBOX) Then

       Me.milesout = _
                DMax("milesin", "dispatchdetails", _
                    "tagnumber = " & Me.VEHICLEBOX)


End If


End Sub