When are sats and when are credits used for zaps?

If the sender and receiver both have wallets attached, the zap exceeds both of their dust limits, and we can predict a successful route with a reasonable fee, the zap prioritizes using the attached wallets and real sats. If either the sender or receiver don't have wallets attached, the zap is below either of their dust limits, or the route to the receiver is too expensive, the zap prioritizes using credits.