System bounds/invariants, ie things that you know can or can't be true. They can be about hardware, os, algorithms, networks, etc. They allow you to quickly pi

System bounds/invariants, ie things that you know can or can't be true. They can be about hardware, os, algorithms, networks, etc. They allow you to quickly pinpoint problems, and avoid exploring places you shouldn't explore ( which is very handy when designing a new system) .