According to the Cities rules, players can lose coins and incur debts. The rules mention that a player can either pay coins or take Debt tokens.
Does this allow a player to choose to take Debt tokens in lieu of paying coins in general (for example, if a card has coins in its cost)?
Alternatively, is it only possible to incur debts when a card explicitly requires a player to discard coins?
You may only take Debt tokens if a card with the Debt icon would cause a loss of coins for you.
So you may not decide to take Debt tokens instead of paying for a building (or for anything else).
The rules for the Cities expansion explain the Debt mechanic on page 4. The first paragraph starts with "A card with the [Debt icon] symbol", and no other condition is mentioned.
The FAQ (also linked from the answer linked above) contains this Q & A:
Q: Can a player chooses to gain debt tokens?
A: No, only the black cards with this symbol [Debt icon] can cause players to incur debts.
