Does what it says on the tin - very brief run-down on the elements of a waypoint and what the various modes mean.
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ArmA 1.16B
Sentry: Wait at this Waypoint until you spot an enemy you're able too attack. Only then proceed with the next waypoint(s).
Guard: Move out to "Guarded by" triggers. Select trigger priority by placement order and presence of other units in guard mode.
When any friendly unit does spot an enemy a fitting counter force is assembled from all units in guard mode and send out hunting. Afterwards they spread out for the triggers again.
http://www.ofpec.com/forum/index.php?topic=883.msg6745#msg6745
http://community.bistudio.com/wiki/ArmA:_Mission_Editor#Waypoint_Order