Terraforming Mars · Board game rules
What is the difference between cards in play and cards in hand?
Quick answer
Cards in hand are unplayed cards you've bought but not yet paid to activate, while cards in play are those you've already played, placed face-up (or face-down for events) with active tags/effects that count toward scoring.
Details
Cards are first bought into your hand during the Research phase for 3 M€ each, and there is no hand limit — these cards produce no effect yet and are simply held privately [p.8 — Generations]. To actually use a card, you must play it from your hand as an action, checking requirements, paying its cost, and resolving its immediate effects [p.9 — Actions].
Once played, cards are placed differently depending on type: automated (green) cards go face up in a stack showing only the top row — no further effect, but their tags remain active; active (blue) cards are kept fully visible since they have ongoing effects or actions you can use each generation; event (red) cards are placed face down in a personal pile after being played, and their tags only apply at the moment they are played, not afterward [p.10 — B) Use a standard project / Place the card].
Only cards in play (and their played events) count for tags used by other cards (e.g., building/space tag discounts, requirements like "science tags") [p.15 — Tags]. At game end, VPs are only counted from cards you have played (including face-down events), not from cards still sitting unplayed in your hand [p.12 — Final scoring].
Watch out: Cards still in your hand at game end give you no VPs or tags — only playing them into your tableau makes them count, except for the Planner milestone, which specifically rewards having many cards in hand rather than in play [p.11 — C) Claim a milestone].