Why do JSON and PDML exports have different data from the same session?

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For example, the PDML file contains information from the "info" column

I don't see that in the PDML output I just got from TShark; it's generated from the packet detail information in the packet detail pane of Wireshark and the -V text output from TShark, not the packet summary information in the packet summary pane of Wireshark and the default text output from TShark.

That does include all layers of the packet detail information, including the top-level layers which usually have summary information such as the type of packet.

With XML, every single item in a hierarchical representation of data can have arbitrary properties, including the "showname" property in PDML, so the top-level layers can have that display information.

This same information is absent from the .json file. Why is that the case?

JSON objects are "an unordered set of name/value pairs". A value can be a string in double quotes, or a number, or true or false or null, or an object or an array". A value cannot simultaneously be a string and an object, and a name cannot have multiple values associated with it, so, for example, the top-level item for a protocol is an object, with entries for all the items below it; only the "simple" items at the bottom of the tree can have string or numerical or Boolean values, everything else has an object as its value, containing the items below it, so there's no place in JSON to put that information.

(JSON is not the best fit for a Wireshark-style packet dissection; it also has trouble handling a representation such as

Foo_protocol = {
    foo_address = 127.0.0.1;
    foo_address = 192.168.42.85;
    foo_comment = "This is the first foo";
    foo_comment = "This foo has two addresses";
}

where a given protocol may allow more than one instance of the same field. XML has fewer restrictions, so PDML could be defined as an XML document type and represent the dissection.)