Claim Status Category codes indicate the general category of the status (accepted, rejected, additional information requested, etc.) which is then further detailed in the Claim Status Category Codes.
A1: Acknowledgement/Receipt-The claim/encounter has been received. This does not mean that the claim has been accepted for adjudication.
P0: Pending: Adjudication/Details-This is a generic message about a pended claim. A pended claim is one for which no remittance advice has been issued, or only part of the claim has been paid.
Please visit http://www.wpc-edi.com/reference/codelists/healthcare/claim-status-category-codes/ for complete list of Claim Status Category Codes