M0 is the heaviest ship class. They are used as a story element, where their purpose is to destroy planets - or rather to be stopped from doing that by the player.

A Xenon M0 appeared in X: Beyond the Frontier. In X3, its remains can be found in Xenon Sector 101.

In Albion Prelude, the ATF Valhalla is, canonically, an M0 Mothership, combining the firepower of a M2+ destroyer with the force projection of a massive carrier.