Register is a sequence of sounds with the same colour and timbre, obtained by keeping the tongue backwards and the larynx down (Fig. 5/5A).
Before dealing with register transition we need to distinguish between chest notes and head notes, that can be identified with their resonances.
Chest resonances (always keep in mind the attack in the mask) develop also in the rib cage and they are also produced in speaking. You can feel them, if you put a hand on your chest.
Higher tonalities develop head resonances, together with chest resonances (always follow the air and use the mask).
Register transition is the passage from chest notes to head notes. This passage happens at a precise point on the scale, that varies depending on the type of voice.