Can someone explain in simple terms what my textbook is saying:

"A 'safe harbor' is provided if the business either treats inventory as non-incidental materials and supplies or treats inventory the same as on the business' financial statements or books and records. The tax accounting rules for non-incidental materials and supplies permit a taxpayer to deduct the cost of such supplies in the tax year in which the supplies are first used or paid for, which ever is later."

I thought you are always supposed to treat inventory as non-incidental, that's why you count inventory, and report it the same way as on your financial statements/books and records? Wouldn't any inventory be non-incidental since you are supposed to track your inventory throughout the year?