I have been asked to create a custom field at “Item by warehouse” level (so really item by location) but I have not been able to figure out how to do this. Would appreciate any quick tips to point me in the correct direction. Essentially for every item at each location we want to have a custom numeric field that we set and that will feed into our demand planning software.

AFAIK you can't add fields to the item's location sublist. EDIT: see answer below We've emulated this in the past with a child record that has location + the other field.

Enable "Advanced Item Location Configuration feature" which allows you to create the field you want using "Other Record Types". I would suggest that you try the advanced feature in Sandbox first. It does move quite a few fields around which might impact some of your searches. Refer: https://netsuite.custhelp.com/app/answers/detail/a_id/67359https://netsuite.custhelp.com/app/answers/detail/a_id/67359