<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.
If there is no <switchmix/> element given in the first <waypoint>, air should be assumed to be the breathing gas.
(see also <waypoint> e.g.)
<samples> <!-- the first waypoint of a profile --> <waypoint> <depth>0.0</depth> <divetime>0.0</divetime> <divemode type="opencircuit"/> <switchmix ref="air"/> </waypoint> <!-- here follow many other waypoints --> </samples>