Longchenpa is the writer who turned the Nyingma inheritance into a body of systematic thought. The school he belonged to held the oldest material in Tibet, the tantras translated in the imperial period, together with the treasure texts recovered later and the Seminal Heart traditions of Dzogchen, and it held them as an accumulation rather than as a system. The newer schools, better organised and better connected politically, were in a position to dismiss the whole inheritance as unscholarly.
His answer is the group of works known as the Seven Treasuries, together with a set of trilogies, in which the whole range is set out in order, argued philosophically and connected to the wider Buddhist scholastic tradition. The result gave the Nyingma a defensible intellectual position it had not previously had, and it remains the reference point: later Nyingma writing is largely commentary on him.
The teaching at its centre is the Great Perfection. Its claim, stated at the public level, is that the ground of the mind is already pure, complete and self-knowing, that nothing needs to be constructed, and that practice is therefore recognition rather than achievement. The instructions by which that recognition is introduced are transmitted personally, by a teacher, to a prepared student, and the atlas notes that requirement rather than describing the instructions, because the requirement is part of what the tradition means by them.