The columns values are transformed to JSON objects for each row. When there are conflicting keys, the result is undefined, might vary between different versions of KNIME.
An example transformation:
From table:
with custom key/values: const and val , Main as data bound key, Num with manual num key and text as automatic:
{"main1":{"text": "Hello", "num": 2, "const":"val"}}{"main2":{"text": "World", "num": 1, "const":"val"}}