It's a good question but there's not a "not applicable" option for specifications. If you want to require it, it has to be numeric. You could potentially use a value (like 0) to represent "not applicable" but if 0 is an allowable value then that would be a problem. On assets you could remove specs that aren't needed, but that can sometimes be more confusing (why some assets show spec X but others don't even though they share the same classification).
I'd consider submitting this as a RFE. I think this could be useful so you could "require" something even if it won't always be set to a value. It'd have to be a separate YORN field and then just check that the spec is provided or that the checkbox is selected that it's not applicable.