If you have the refueling point set to the left side of the aircraft, you would call for refueling first, and whilst you're getting refueled, you would call for boarding. If the refueling point is set to the right side of the aircraft, the cargo won't get loaded whilst refueling is taking place because the vehicle paths for the fuel truck and cargo loaders can interfere with each other, so the cargo loaders won't move until the refueling is complete. Likewise, if the refueling point is set to the left side of the aircraft and you board the aircraft using stairs, you might run into that same issue.