The general rule of thumb is that the recommendation should address some aspect of your life that won't/can't be covered by your other recommenders, typically because they have no knowledge of it.
SOME examples (in addition to the excellent one cited above) would include:
You've worked a part-time job throughout h.s. to support your family and thus have very limited opportunity for ECAs and sports. A letter from your employer would be helpful.
Your primary ECA is outside of school and you've done fantastic things in it (e.g., raised a record amount of money in a fundraiser). A letter from the organization leader would be helpful.
A letter from a neighbor, family friend, another teacher, coach, pastor, etc. saying you're a terrific person isn't going to make any difference.