Well, since there's an interface forum, and there's no mention of this feature in the design doc so far as I can tell, and since it's one of my favorite features:
AIW1 has a feature where you can prevent spending on a control group unless you have at least X metal. EG:
* You set a space dock to be in control group 1
* You set the space dock to loop build fighters (which are automatically put into group 1)
* You go into CTRLS and set X to be 100k
If you have less than 100k metal:
* The space dock won't make fighters
* Any fighters in group 1 that are damaged will be ignored by your engis
Otherwise, everything works as normal.
Things you can do with this:
* Always keep some metal in reserve for if you need to build something in a hurry (like a pile of turrets in a new colony). If all your constructors are in control groups with X set to at least 200k, then even if you take heavy losses and need to refleet, you'll always keep at least 200k back for plopping down turrets.
* Channel excess metal into big projects: put your broken golems into control group 9, and set X to 1.9 million metal. Put your highest priority golem into group 8, with X at 1.8 million. Now you can leave all your golems set to build, any excess metal you have will go to building your next golem, and once it finishes, your excess metal will instead flow to the rest of your golems.
* Deprioritize less critical ships like raid starships and scouts. Maybe I want to build triangle ships, then frontline starships, then raid starships, then scouts; just put their constructors into control groups and set their X values to 100k, 200k, 300k, and 800k, and they'll always be built in that order.
It's a bit clunky tabbing through all the different control groups, though. I think having one page for all the control groups would be easier. And rather than an absolute metal number, having a slider for a percent of total metal to reserve would be easier to use. IE, you can tell control group 1 to suspend spending if you're at less than 5% of your total metal.