Can units be played as reactions (e.g., with 'flash' or when flipping a hiding unit), given that Rule 540 doesn't explicitly mention playing units during a chain?

Ruling: If a card has text allowing it to be played as a reaction (like hidden units), you follow the card text via the golden rule. A unit with such text can be played whenever you have priority during a chain and resolves immediately with no player gaining priority to react to it.

Sequence:

• Play the unit when you have priority during a chain (or as the first item)
• The unit resolves immediately
• No player gains priority to respond to the unit being played
• The chain continues normally

Nuances:

• This is different from an ability that makes you play a unit, which functions normally on the chain with players able to respond
• Playing a permanent is equivalent to activating and resolving a spell that no player can react to
• Rule 540.1 could be clarified to say "Play a Card that is legally timed" since timing restrictions already prevent playing most cards during showdowns