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 DAX is
A DAX DATATABLE expression that defines a table inline in a Power BI model.
Power BI report authors.
What changes when you convert Plain Text to DAX
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.
All columns are declared as STRING. Change the type declarations by hand if you need numeric columns.
What carries over from Plain Text to DAX
DAX can express everything plain text holds about a table, so this conversion is about shape rather than loss.
DAX 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.
DAX marks its header cells differently from its data cells, so the first row is written with that marker.
The identifier 007 comes out of the DAX 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 DAX file goes wrong.
The quotes around Jonah "Jo" Pryce are escaped with a backslash in the DAX.