There is much confusion over the word “mix”. This makes sense, since most people do not know the difference between chest register and head register except in terms of range. Chest is low and head is high.
This is the most simplistic way to view registration and, in a skilled singer, it is no longer true that registration is dependent upon pitch range. In fact, head register as a quality, can be taken low in pitch, chest register can be taken up in pitch, and both registers should respond to volume change. Louder sounds will be chestier and softer ones headier, if all other factors are equal. If, however, you have been taught to focus only on “resonance” or “placement” and “breath support” you will not understand register independence, because your sound could be an amalgamation of whatever was there, maneuvered into whatever resonance you could find or manage. This is not necessarily a recipe for bad vocal function but it is likely not a way to sing in a variety of styles, colors or qualities for expressive or musical purposes.
If you can find a strong, chesty belt that is easy to produce and costs you nothing in terms of vocal effort, and you can also sing a light, easy free head register sound that is sweet and clear, you will have no trouble determining what mix is. You will find mix when the middle pitches, on their own, without extra help of any kind, combine into a nice round solid sound that you can easily control in terms of volume and quality. Mix, generally, is therefore anything that is not absolutely chesty chest or belt, or heady head (or falsetto in men).
Determining if something is “chest mix” or “head mix” is of no consequence unless the music is such that it requires a particular default to one of these. In most people who have a mix and can use it, it will be chestier on the louder, lower pitches and headier on the higher softer pitches all by itself. The quality is determined by the content of the lyrics and the pitch patterns that are being sung. Worrying about whether something is head/mix or chest/mix is usually a waste of time in terms of performance. It is not, however, a waste of time if you are stuck in one and need the other and can’t tell the difference. Then, you need to know and know well what’s going on. Of course, this is one of the points of training: to develop a balanced vocal mechanism that is not too much anything but rather an combination of many things. Resonance is not the driving factor, registration is.