The element indicates the so-called "remaining bottom time" in seconds at the respective <waypoint>. The "remaining bottom time" is that time span after which the diver has to begin the ascent so that the surface is reached with the reserve pressure given before.
<waypoint>
<waypoint>
see also <waypoint>
<!-- remaining bottom time 5 min 30 sek -->
<remainingbottomtime>330.0</remainingbottomtime>