What JSON Lines is
JSON Lines is one complete JSON object per line, with no enclosing array and no commas between records. The file is not valid JSON as a whole, and that is the point.
Log pipelines, machine learning datasets and anything that appends records or reads them in a stream.
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 changes when you convert JSON Lines to Plain Text
Each line is parsed on its own, so a corrupt record does not take the file with it. Keys are unioned across lines in first-seen order, so a record missing a field leaves that cell empty rather than shifting the columns.
Columns are padded to the widest value. Width is counted in display columns rather than characters, so a table containing Chinese, Japanese or Korean text still lines up: those glyphs occupy two columns in a monospace font.
What carries over from JSON Lines to Plain Text
JSON Lines records 3 things about a table that plain text has no way to hold.
JSON Lines distinguishes a number from the text of a number; plain text does not, so every value arrives as text and has to be re-parsed by whatever reads it next.
An empty cell and a missing one are the same thing in plain text, so a null and an empty string in the JSON Lines both come out blank and can no longer be told apart.
JSON Lines can hold a value that is itself a list or an object. plain text has only flat cells, so nested values are flattened into one cell rather than being spread across columns.
The keys stop being repeated: plain text writes the column names once at the top, so the same table takes noticeably less space.
The identifier 007 comes out of the Plain Text 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 Plain Text file goes wrong.
The quotes around Jonah "Jo" Pryce are left as a plain quote character in the Plain Text.