Anno 1800
Is the discard-up-to-two-cards population effect mandatory, and must you use it at once?
The rulebook says you may return up to two cards; the must-use-immediately timing comes from consistent forum replies.
It is optional: you may discard zero, one or two cards, and only from your hand. It must be resolved right after you play the card and cannot be saved for later. The card's exclamation marks are the reminder.