Lineups are different card decks you can set up and save. You can set up to 3 different lineups.
Navigate to the lineup menu and you'll there are the tabs: 1, 2 and 3.

Tapping on any of the lineup tabs and changing the cards on any slot will not change the other two lineups. This means you can always select a different lineup and change the cards to test them out without losing your other lineup!