After struggling for HOURS trying to get the correct header on my report and having problems because it had sum[Mileage] as part of the field so I couldn't set it from the form, I created and stacked a header for each of my three categories, but could NOT figure out how to hide the two I didn't need depending on which button I clicked, so I finally asked GPT.  Thier answer worked perfect!

Private Sub btnOpenReport_Click()
    DoCmd.OpenReport "MileageR", acViewPreview
    Reports("MileageR").Controls("KennelHeader").Visible = False
    Reports("MileageR").Controls("MedicalHeader").Visible = False
End Sub