Otherworlder (異世界人 Isekaijin, lit. "Other World People"?), refers to individuals that come from other worlds.
Summoned (召喚者 shōkansha?) refer to those that arrived via summoning rituals. It's a dangerous form of transportation, as the victim's body is exposed to massive amounts of magicules. Only those that bear a strong enough soul can withstand it.
There exist very few forces capable of summoning them because it requires very definite circumstances to be prepared as to allow one to summon a person with powerful abilities, especially those whose will is exceptionally strong. The necessary conditions for a proper summoning are as follows:
- The success rate of the ritual with thirty magicians chanting summoning magic over a period of three days is less than 0.03%.
- The magicians who have just cast the summoning magic will be unable to cast it again for a set interval.
- The cooldown period ranges from 33–88 years, the time during this period is used to prepare a suitable environment to increase the success rate of the summoning.
However, if the summoning were to be conducted under incomplete conditions, it can be done continuously. But the downside of improper summons such as these usually results in children being forcefully summoned with abundant magic infused into them as a result.
Unlike the Otherworlders who are guaranteed to possess a unique skill, the Summoned's rate of success of gaining a unique skill is relatively low, with a curse carved into their souls in the bargain. The rampaging magical energy of the curse within will cause their bodies to implode, fried alive by the very power they possess. The recorded lifespan of those improperly summoned without a unique skill is three years. Therefore, unless a unique skill is obtained, death is certain.
Leon Cromwell is to blame for spreading this knowledge to human kingdoms in order to summon his friend Chloe Aubert; by using the greedy humans by proxy. However, Leon quickly realized summoning will cease upon Rimuru ascending to Demon Lord status; the humans will not risk Rimuru's wrath (or sending Veldora) for kidnapping and enslaving youths from his world.
Stray (偶発的にやって来る異世界人 Gūhatsu-teki ni yattekuru i sekai-jin, lit. "Otherworlder transported by accident"?) or Stranded One or Accidental Otherworlder refer to those Otherworlders who came from Earth (or possibly other worlds) by sheer accident. There is no clear cause or pattern known for why people get transfer between worlds like that, but in general, it's considered a one-way street and the phenomenon is not known to happen into the opposite direction.
Among the three Transmigration types, it's a dangerous way to be transported in the initial stage, since there is no fixed target location. So it's possible to be suddenly transported into the middle of a territory that is hostile to humans, in a completely unprepared state, possibly even into a hazardous region that leads to almost instantaneous death upon arrival. However, if the first hurdle is passed and the Stray manages to find a home, then the greatest danger is passed. Unlike Summons, the Strays are not subjected to an enslavement ritual and thus have greater freedom, however likewise there is no guarantee of obtaining a Unique Skill in turn, and in general, there is a chance for these kinds of Otherworlders to be comparatively weak like in the case of Yoshida, however that is not guaranteed either. For example, Hinata Sakaguchi obtained 2 Unique Skills despite being a Stray.
Reincarnator (転生者 tenseisha?) refers to individuals that reincarnated from another world. Being reincarnated tends to happen randomly, especially those whose souls with strong intentions engraved upon them. Although there are those among the reincarnated who still remember their past life, to have reincarnated from a different world is exceedingly rare. Moreover, one would normally be reincarnated as a human. However, there are very few exceptional cases. For someone to be reincarnated a monster, and from magical energy itself, is something that remains unheard of.
As for those who came to this plane by accident, it also happens randomly. However, unlike those who undergo reincarnation and have to be reborn, they arrive here with their original bodies intact and whole. In addition, they still retain their original memories.
In any case, setting aside whether method they came by, it is a known fact that those who came "naturally" will possess the special intellect and acquire some unique skills, based on the characteristics of the individual themselves, naturally upon their arrival.
It's pretty common that Summoned Ones tend to have a Curse of Obedience etched on their Soul to make them unable to go against the Summoner.