When you use any peels, you usually use a grater and grate them, like in the following image with a lemon peel:
The reason behind that is that the peel and / or skin on its own isn't easily digestible or even yummy. Sometimes, the skin is cooked in a soup or marinade to achieve a distinguishable flavour. (Please note that oftentimes not the skin but rather the pulp is used to do that. The reason here is that the skin usually has a stronger taste and you don't want to overdo it.)
In your recipe, I guess that the peel was grated and put with everything else. It could also be that the peel is cooked within the sauce to give the sauce a mandarin-like taste.
I marked two spots in your image where some kind of mandarin-like ingredients are visible. Either to the left there are small chunks which would come from grating the mandarine peel or on the right there are small stripes coming from slicing the mandarine peel very finely. However, the image doesn't have a good enough resolution so I'm not certain.