Those events will run only run for a specific object so they are optimal and should be your preferred way to register an event.

As you can guess, it is using local variables, for example:
Will cause the myerf_useitem script to be called when the item on which the variable is set is used.

You can also register multiple script to the same event:

With the variables above, all scripts (myerf_useitem, myerf_useitem2, myerf_useitem3 and myerf_useitem4) will run when the item is used.

You can register / unregister a variable event dynamically with the following functions:

void RegisterVariableEvent(object oObject, string sEvent, string sScript, int bNoByPass=FALSE);
void UnregisterVariableEvent(object oObject, string sEvent, string sScript);