I always have to research IRS against any internal policy for the client I am working with. The final regs that got issued were supposed to help clarify and simplify from the mishmash that existed prior to this (I have my notes dated from 2014 proposals to finals). I try to apply safe harbor when something else, such as GASB/FASB, won't apply. For example, I worked with someone that had a $1,000 policy limit and I got their board to raise that to $3,000. Plenty of repairs bump against $3k, nowadays. You seem to be learning about an Annual Election:

https://www.irs.gov/businesses/small-businesses-self-employed/tangible-property-final-regulations#Wh...

"You make the election to use the safe harbor for each taxable year in which qualifying amounts are incurred.

• The election is made by attaching a statement to your income tax return for the taxable year. See When and how do you make an election provided under the final tangibles regulations?
• An annual election is not a change in method of accounting. Therefore, you shouldn't file Form 3115, Application for Change in Method of Accounting, to make this election or to stop applying the safe harbor in a subsequent year."