Weapon of Mass Destruction: Per the Ley Accords (i.e., the Laws and Customs of War), in descending order of aargh, you’ve got star-killing weapons (nova bombs, including specifically star-targeted strangelet bombs, twist-pinch bombs, and most hypothetical causal weapons), planet-killing weapons (large/fast kinetic impacters, including asteroid drops, planet-targeted strangelet bombs, and relativistic k-kill weapons, extremely large [strategic-plus] energy-burst weapons, including nucleonic and antimatter warheads, and self-replicating planetary-scale war machines [berserker probes]), and uncontrollable self-replicating infoweapons and memetic weapons (that affect systems beyond their legitimate targets, propagate themselves widely across the extranet, and lie dormant in archives to come out and kill innocent people ten thousand years later), and ecocidal weapons (merely large [strategic-plus] energy-burst weapons or ongoing bombardments with same, general bombardments with small kinetic impactors [smaller asteroid drops, de-orbited satellites/stations, or orbital k-kill systems], uncontrolled self-replicating weapons [autonomous goo, unchained bioweapons, technophages, and clanking replicators], global ecoweapons and phage weapons, or the use of persistent ecoweapons and bioweapons, salting nucleonic weapons [say, cobalt bombs], or chemical weapons likely to permanently damage or accumulate in ecosystems).
Using any of the first three types anywhere, or the fourth on a garden world, will get your entire polity blasted and governance wiped out even if it takes the use of otherwise prohibited technologies to do it; these are technologies that eliminate habitable worlds – and those are really goddamned expensive – or tend to run beyond any reasonable control. Ergo, they’re the galaxy’s primary do-not-fuck-with list.
Mere tactical-to-strategic nucleonic/antimatter weapons, non-persistent chemical and biological weapons, incendiary weapons, cerebroergetic weapons, and nanoweapons are not covered by this treaty, or considered the equivalent of WMDs. Not enough mass. They’re all fair game.