Missions are a core gameplay system in Trace/ALTER wherein players can accept a mission graded based on its difficulty at a mission terminal and then use a metro terminal to be sent off to an instanced mission location with a set of objectives they must clear for a reward of experience, yen, and possible loot.
Missions are assigned a letter grade which determines both the reputation required to accept the mission as well as the relative difficulty of the mission. E-rank missions are the least difficult missions, with no risk of invasion by ALTERs, minimal chance of enemies appearing (unless the mission description involves phrases such as “maintain item integrity” or “threats still present on site”), and no risk of serious injury. On the opposite end of the spectrum, A-rank missions are the current highest difficulty missions, often containing boss tier enemies, being susceptible to invasions, and the possibility of player death if things go incredibly wrong.