Hi,
Redlining as far as I know has at least 2 meanings.
In terms of equipment the redilining is taking it to the max. Driving it to its maximum permissable tolerance. Simular to driving a car with the revs bordering on the red zone.
As for print, redlining is just that. Highlighting areas for review/correction or deleting by underlining them in red. This used to be on paper but can be on electronic print allowing the author (an in some cases only the author) to correct any errors.
Regards