I think "alive unit" returns true if the group's leader thinks the unit is alive. It can take a while for them to notice, especially if you kill off a whole group at once. Similarly, the list of units in a group isn't updated until the leader acknowledges the unit is dead.
It looks like "damage unit" returns the actual amount of damage it's received, so if you want to know whether a unit is actually alive or not, you can use if (damage unit < 1.0). nomdeplume 12:39, 16 November 2009 (CET)
Returns 'BOOL' if the object does'nt exists. For example in multiplayer when a object is removed alive will return 'BOOL' if a new player join after the object were destroyed.