Report To Open Based On Comobox Selection

I have some code that was written by someone else. I am trying to duplicate this in another database but, can't seem to get it to work. I also don't understand how this code is working as as there is no reference to the combo box in the code behind the print button. But depending on what you select in the combo box the report opens and filters on that selection. The following code is behind the print button. There is a separate combo box, that does not seem to have a connection to the print button. It just has a query behind it for the items to select from.

Public Sub BTNPrint_Project__Job__Numbers_Click()
On Error GoTo Err_BTNPrint_Project__Job__Numbers_Click
Dim stDocName As String
stDocName = "RPTProject Number (Job) Book Year"
DoCmd.OpenReport stDocName, acPreview
Exit_BTNPrint_Project__Job__Numbers_Clic:
Exit Sub
Err_BTNPrint_Project__Job__Numbers_Click:
MsgBox Err.Description
Resume Exit_BTNPrint_Project__Job__Numbers_Clic