Sort of from the Prima guide bit on WPs:
DESTROY: this WP must be placed ON a target unit - select attacking unit, select Waypoints - double-click on target unit. Attacker moves to WP, attacks when he sees the unit, if not searches for the unit and kills it.
The implication is the WP is complete when the target unit is no more. A Seek & Destroy is too general as THobson says - the group will wander around looking for anyone to kill until they get bored.
The snag is that it suggests it works for a single unit rather than a group, but it's a safe bet the attacking group will take out the target unit's group as well.
The
{alive _x} count units groupname < 1
condition will hold the attackers at the WP until the targets are dead but if the target group bimble off elsewhere, the attackers won't go after them and may sit on the WP forever.