Some people use the singing terms belt and chest voice as if they’re the same. Belt and chest voice may be related, but they’re not the same. Belt uses some chest voice, but it’s not pure chest voice. Mix just means that you’re mixing or combining registers — combining or using head voice and chest voice at the same time and using qualities of both.
Two primary muscle groups help create head voice and chest voice. When you sing in the middle part of the voice or mix, you’re using both of those muscle groups at the same time. If you sing in chest voice, you’re using only one of the muscle groups; the one that helps you create chest voice.
One of the reasons the term mix belt has become popular is to help people understand that belt is not pure chest voice — it’s a combination or mixture of registers used at the same time. Mix belt is some chest voice and a lot of forward resonance to create a belt.