Does a digivolution requirement that digivolves from a Tamer mean that the Tamer that will become a digivolution card is treated as a Digimon when it digivolves?
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No, it digivolves from the Tamer as-is. It isn't treated as a Digimon that digivolves, therefore "when a Digimon would digivolve" effects and "when a Digimon digivolves" effects don't trigger. In addition, this digivolution requirement can be used to digivolve a Tamer even if a "Digimon can't digivolve" effect activates.
Source: BT18-049 Zephyrmon