What Plain Text is
Plain text here means columns lined up with spaces, the way a command line tool prints a table.
Terminal output, log files, fixed-width exports from older systems.
What JSON is
JSON is a text format for nested data: objects with named keys, arrays, strings, numbers, booleans and null. It has no date type and no comments.
Every web API, most configuration files and anything that talks to JavaScript.
What changes when you convert Plain Text to JSON
Two shapes are handled. If the same character positions are blank on every line, those gutters are treated as column boundaries, so values containing single spaces stay intact. Otherwise the lines are split on runs of whitespace.
You choose the shape. An array of objects is the default and the one most code expects. A 2D array is smaller. Column arrays suit charting libraries. A keyed object suits lookups by id.
What carries over from Plain Text to JSON
One property of the plain text has no home in JSON, and it is worth knowing which before you convert.
JSON wants a type for each column, and plain text does not record one, so each column is typed from what its values look like. A column of digits that should stay text - a zip code, a phone number, a leading-zero id - is the usual thing to check afterwards.
JSON has no header row. The column names are repeated as keys on every record instead, which is why the result is bigger on disk than the table it came from.
The identifier 007 comes out of the JSON as 007. Reading it as a number would have made it 7, and an id that changes value is worse than one that stays text.
The role "Analyst, data" survives with its comma, in one cell rather than split across two. That is the first thing to check in any converted table, and the usual place a JSON file goes wrong.
The quotes around Jonah "Jo" Pryce are escaped with a backslash in the JSON.