Basically a focussed balance mod - you pick an area that you try and fix the gameplay/balance with by making a tight small mod that makes the least possible changes. Once the mod is working well it can basically be inserted directly into FAF - rather than some of the more sweeping scale mods that change air/navy/land all at once where the flow-on effects are more far-reaching and complicated to predict.
Eg. my first leverage mod is Air Leverage - it only addresses t2 and t3 air units & air counters with the goal of improving air gameplay. As mentioned I'll work on a seraphim leverage mod where I'll try and make seraphim more interesting and unique, increase their tactical and strategic options in t2/3.