Sideline is a term used to describe when a card in play has expended its uses or is otherwise unusable. This is demonstrated on the game board by turning the card sideways instead of upright.
Example Image
Left: Upright. Right: Sidelined.
Some Effects Cause Cards to Become Sidelined
Plot Twists Sideline Agendas and Tactics
Most commonly, all Plot Twist cards have an effect that causes Agendas and Tactics to become sidelined, meaning that can’t be used that turn.
Tip
That means you should activate a Plot Twist on the opponent’s turn so their Agenda cannot Proselytize at the end of their turn!
Creatures Get Sidelined After Using Them
If, on your turn, you expend all the actions they can perform (depending on stat-based capabilities).
Example
The TK Knight in the image above has SWG below 5, so it cannot perform attacks and abilities in the same turn. However, both its FYT and NRG are at 5 or above, meaning it can do two attacks or two abilities in one turn before being completely exerted. Let’s say you use the TK Knight to do two STR-based Physical Attacks. After the second one, you would sideline the Creature to show that it cannot be used anymore that turn.
Important Rules
Sidelined Does Not Mean Out of Play
A sidelined Creature in the Active Zone still blocks the way to a player’s Think Tank. You must kill them or otherwise remove them from play before reaching the Seated Figurehead or other cards in the Think Tank.
Connection to Magic: the Gathering
This is obviously my namechange for tapping cards in magic.