Heuristics are rules of thumb. They are generally true, but there can be exceptions in certain contexts, so they are more flexible than rules.
System boundaries can be put in place by providing heuristic rules to govern a particular principle.
For example we often use these three for teams:
- Increase feedback
- Decrease batch size
- Pull work through the system
See the Path to Mastering Skills article for more some more in depth exploration of when heuristics are appropriate.