In this document, the E3 included an explanation of the relationship between the MTCR Category I definition and nuclear weapons.
According to the MTCR Guidelines, Cat I items
include complete rocket systems (including ballistic missiles, space launch vehicles and sounding rockets) and unmanned air vehicle systems (including cruise missiles systems, target and reconnaissance drones) with capabilities exceeding a 300km/500kg range/payload threshold;
The August E3 letter to the UNSC explains that the 300/500 thresholds are “the recognized minima for the mass of a nuclear warhead and the distance required to ensure self-preservation after delivery.”