If you choose to mark a deal as lost, you actually state that the deal has not gone through for a specific reason.
- You can edit those reasons under Settings > Deals > Preferences > Manage deal loss reasons.
- Lost deals are also taken into account in the statistics.
It's better to only delete a deal if you're sure that there has been a mistake. A deleted deal is not taken into account in the statistics.
If you deleted a deal by mistake, you can still retrieve it using these instructions.