2020-11-10 : Bell Borca notes mana values only if it's on the battlefield as a card is put into exile.

2020-11-10 : If a card in exile has {X} in its mana cost, X is considered to be 0.

2020-11-10 : If you don't play the exiled card, it remains exiled.

2020-11-10 : Look at the card in exile, not its previous zone, to determine its mana value.

2020-11-10 : While not on the battlefield, and before any cards are noted for it each turn, Bell Borca's power is 0.

2020-11-10 : You must follow the normal timing permissions and restrictions for the card exiled by Bell Borca's last ability. If it's a land, you can't play it unless you have land plays available.

2020-11-10 : You'll still pay all costs for a spell cast this way, including additional costs. You may also pay alternative costs if any are available.