If you see a +20 degree score that would certainly indicate that the bag should be cozy enough for temperatures that are 20 levels or warmer.. And if you see a - 20 level score then it has actually been figured out that the bag needs to be cozy sufficient for minus 20 degree climate.
The other sacrifice is the size or thickness of the bag. A mummy sleeping bag essentially will certainly be much smaller sized for bring around which is why it is a preference for many backpackers/hikers. The rectangular resting bag isn't as small yet because it's not common to be lugging it, apart from somewhere in a vehicle while getting to your intended location, the bulkiness of the bag is not as large a problem.
Mummy bags are the tiniest resting bags as well as are very light to carry, making them optimal for those hiking trips. Lots of people could find these bags uncomfortable as they are really limiting, but they are the much better choice for outdoor camping during cool weather condition seasons because of their heat, and also hiking journeys because of their tiny dimension.