The National Alliance for Broader Impacts has recently put out a “Guiding Principles and Questions for National Science Foundation Proposals” informational PDF.
https://broaderimpacts.net/wp-content/uploads/2016/05/nabi_guiding_principles.pdf
It provides great info if you’re thinking of broader impacts for a NSF proposal as well as if you’re thinking about broader impacts for any other purpose!
There is also a push for NSF review panels to use the NABI handout as a guideline when reviewing grants, so it’s worth checking out!
