The NSF is divided into broad directorates like Biological Science, Engineering,Social/Behavioral/Economics Science, etc. These are divided into divisions, which are further subdivided into programs or fields of study. For example, the Math and Physical Science directorate includes divisions of Physics, Mathematics, Chemistry, etc. Within the Chemistry Division are programs in Chemical Catalysis, Chemical Synthesis, etc.
You can find a list of field’s and GRFP’s advice about picking one here.
Some projects fit clearly into one field, others could fit in multiple fields. For example, a new algorithm for doing computational chemistry might be presented as a chemistry project, a staistics project, or a computer science project. Similarly, a study of forced sterilization of women in prisons might fit in Law and Social Science, Medical Anthropology, or, if framed in context of the eugenics movement, even in the History and Philosophy of Science.
Your project might fit clearly into a dominant paradigm of one field or be peripheral to another—reviewers in the first would immediately see the relevance while reviewers in the second might be nonplussed. Another project might be normal science in one field but transformational in another. Put serious thought into your choice!
HERE’S HOW TO CHOOSE:
It’s easy for most folks in the GEO, ENG, CISE, BIO, and CHEM directorates — the field divisions are finely grained.
Visit the program pages on the NSF website. Read the description, then follow the link at the bottom of the page to see projects that program has funded. Download the data to an excel file and look over the titles and abstracts. If you find a clear fit, take it.
2. Avoid the “other“ categories if possible.
Of course, sometimes you can’t avoid it: For example, there is no “entomology” field, so if you are doing a taxonomic research project on an insect family, you’d have to submit as “Life Sciences, other,” and then write in “entomology.” There are enough entomologists that the program officer will create a panel for you. However, if you were merely using the insects as a model system for a genetics or neurocience project, you should probably submit in the “Life Sciences, Genetics” or “Life Sciences, Neuroscience” fields.
3. Recognize that it is generally harder to frame an interdisciplinary project (two or three fields) in a way that speaks clearly to people from multiple disciplines. It’s best to do this only if an interdisciplinary paradigm is clearly established – e.g., neuroeconomics.
Discuss strategy with your advisor!