<switchmix/> indicates the breathing gas used. It's possible to change the breathing gas during a dive as often as necessary, but only one <switchmix/> statement may occur inside a <waypoint> element.
The first <waypoint> of a dive profile must contain a <switchmix/> statement — to indicate the breathing gas used during this dive (or at least to determine the starting breathing gas).
(see also <waypoint> e.g.)
<samples> <!-- the first waypoint of a profile --> <waypoint> <depth>0.0</depth> <divetime>0.0</divetime> <switchmix ref="air"/> </waypoint> <!-- here follow many other waypoints --> </samples>