For the latest experimental build:
You can create an argument of type AT_SysVar. This is used for the graviton condensor. The hook says:
The :7000000 part specifies the default value if this ability is not added by a subsystem.
In the hook, it is used like this:
double amt = dmg_per_second.fromSys(abl.subsystem, efficiencyObj=abl.obj) * time;
The fromSys() call on the argument takes the subsystem stored in the ability (this can be null, but fromSys() deals with that), and determines the value to return based on the subsystem variable or the default passed in the AT_SysVar argument. The efficiencyObj argument is optional, and if specified the fromSys() function will also calculate the efficiciency due to damage and modify the value by that.
This is better than getEfficiencySum because it's cleaner, and because it only takes the value from the subsystem being used, not from all subsystems that have that variable (so two separate subsystems won't use the sum value of their variables, but each gets calculated separately).
Note that this only works if the ability is added through an "Ability:ID" tag in the subsystem, not if it's added through any hook. If it is added in another way than with a tag, it will just use the default value specified in the argument.