Passes (the tickets and vouchers you have already paid for) are kept together on the trip's Passes card: city passes, theme-park and tour passes, transit passes, airline and hotel credits, and gift cards.
A pass is single-use, multi-use, or balance-tracked, and a balance-tracked one has its remaining balance come down as you spend it. Two clocks are kept for you: the fixed expiry date, and a countdown you start yourself at activation, so a 48-hour city pass only begins counting when you activate it. Anything that has run out of time is flagged automatically.
Link a pass or voucher to the activities it pays for, and the redemptions, partial or full, and the remaining balance are tracked and flow into the trip's cost tracking. Passes are free on every plan, with no cap on how many you keep.
Passes sit with the other three travel-day tools on How it works.