Generally, monsters are summoned by summoners from the same splinter as they are.
However, neutral monsters can be summoned by any summoner, from any splinter, and Dragon summoners can summon dragon monster cards, as well as monsters from the splinter selected at the beginning of the team drafting phase.
Lux Vega is the first neutral summoner that can summon monsters from the selected splinter, similar to dragon summoners, except it can't summon dragon monsters.
Related: Splinterlands Website
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