As the document explained, there's a checksum parameter in addResultData(result, tag, checksum, own=True). is PDG use it for checking files version? How can I make use of it? cause I find the built-in nodes' source code, the checksum parameter is set to 0 for many nodes.

It's not really used internally. However the idea is that users can make use of it with their own logic to track if files have changed by setting it to (for example) the mtime of the file, or a 64 bit hash of the file contents. I believe the only stock node that does this is `filepattern`.