Plans belong to venues
Every event held in the room sells the same chairs.
In the paid plans
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A hall is a picture of a room, and it belongs to the room rather than to an event. A cinema with two screens draws two halls and then never draws them again.
Every showing in Screen 1 sells the same chairs, so a new event in a room you already have is a choice from a list, not an afternoon of drawing.
More in Seating
Draw the room once and sell the chair rather than the number.