I get asked on a regular basis what the difference between a contact hour and a PDU is. When people come to my PMI-ACP exam prep class, they qualify to claim 21 Agile contact hours. If they currently have another PMI credential, they could choose to apply those 21 hours as a PDU.
PMI Agile Contact Hours
When completing your PMI-ACP application, you are required to report (among other things) your "Agile" education. They will be referred to and measured as contact hours. To qualify to sit for the ACP exam, you need 21 contact hours.
Professional Development Units (PDUs)
PDUs can only be applied if you have a PMI credential. If you try to claim a PDU and you don't have a credential, PMI will politely either tell you don't have permission to that area of the website (where you claim the PDU) or they will send you a friendly email. The image below is only viewable if you have at least one PMI credential.
Hope this brief overview helps. If you have any questions, please leave a comment below.