object addEventHandler ["varChanged", {
params ["_varName", "_oldValue", "_newValue", "_oldPubValue", "_newPubValue""];
}];

So if a player or server setVars on a object, players on clientside can be informed of change and vice-versa

_oldValue/_newValue would be the var data before and after change]
_oldPubValue/_newPubValue would be the public param before and after change. Like if it went from -2 (all clients) to only server (2)