Where can I get the official document about pdml file format?

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Here is the original on the Internet Archive (PDML Specification) that is referenced in the source of the commit Add the ability to print packet dissections in PDML (an XML-based format)

Notes in Wireshark source tree README.xml-output - Protocol Dissection in XML Format

A higher level paper discussing PDML and PSML: Using XML for Efficient and Modular Packet Processing
Keywords: Network Packet Processing, Modular Packet Processing, XML, NetPDL, PDML, PSML.

(PDML page on the Wireshark wiki)