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 Protobuf Schema is
A proto3 message definition with numbered fields.
gRPC services and anything using Protocol Buffers.
What changes when you convert Plain Text to Protobuf Schema
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.
Field numbers are assigned in column order starting at one. Types are inferred from the values.
What carries over from Plain Text to Protobuf Schema
One property of the plain text has no home in Protocol Buffers, and it is worth knowing which before you convert.
Protocol Buffers 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.
Protocol Buffers 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.