There are four laryngeal vibratory mechanisms. Mechanism 1 (M1) is usually associated with chest voice, whereas mechanism 2 (M2) is usually associated with falsetto and head voice. Mixed voice can be done in either mechanism (is it a “chesty mix” or a “heady mix”?). These mechanisms are defined physiologically and not acoustically (though they have acoustic implications), therefore, they can be objectively qualified.
As you do a glissando from chest voice to head voice, you’ll course through the ‘mix’, where you’ll have to pay attention to the sharp transition from M1 to M2 so as to mask it. This transition is sudden and not gradual, however, by lightening up M1 as you go higher in pitch, this transition can be masked (though it still happens).
Further reading: https://hal.archives-ouvertes.fr/hal-00207991/ and https://cramdvoicelessons.blog/encyclopedia/laryngeal-vibratory-mechanisms/