The IRS provided incrementally helpful guidance on the types of costs that must be capitalized in September. The capitalization requirements are generally more restrictive than GAAP where only ~30% of costs are capitalized for book purposes at many companies. There are some types of software development that are outside the scope of 174 though such as UI changes that don't add new capabilities.
https://www.irs.gov/pub/irs-drop/n-23-63.pdf