Some Mahayana traditions, particularly in the Tibetan scholastic presentation, teach not three marks but four dharma seals, adding to impermanence, suffering and non-self a fourth: that nirvana is peace (shanti), and the addition, though it looks like simple expansion, addresses a genuine gap the three-mark formula the main entry describes leaves open.
The three marks as stated are entirely negative in one important respect: they describe what is wrong with conditioned existence, impermanent, unsatisfactory, without a findable self, without directly stating what the cessation of that condition is actually like or why anyone should want it beyond the relief of a diagnosed problem. The fourth seal supplies the positive counterpart, asserting that nirvana, the unconditioned toward which the whole analysis points, is not merely the absence of suffering but a condition properly described as peace, addressing a concern that a purely negative teaching risks sounding like nihilism, a worry closely related to the caution developed elsewhere in this collection regarding how Madhyamaka handles the danger of emptiness collapsing into nihilism.
Tibetan doxographical presentations, particularly in the Kagyu and Nyingma traditions' teaching manuals, treat the four seals as the minimal criterion distinguishing a genuinely Buddhist teaching from a non-Buddhist one, holding that any system affirming all four qualifies as authentically Buddhist, a boundary-marking function beyond their descriptive content: not merely a list of features conditioned existence has, but a checklist for identifying Buddhist teaching across the tradition's enormous internal diversity of school and practice.