Interest allocation rules.
The tax treatment of interest expense differs depending on its type. For example, home mortgage interest and investment interest are treated differently. “Interest allocation” rules require you to allocate (classify) your interest expense so it is deducted (or capitalized) on the correct line of your return and receives the right tax treatment. These rules could affect how much interest you are allowed to deduct on Schedule C or C-EZ.
Generally, you allocate interest expense by tracing how the proceeds of the loan were used. See chapter 4 of Pub. 535 for details.
If you paid interest on a debt secured by your main home and any of the proceeds from that debt were used in connection with your trade or business, see chapter 4 of Pub. 535 to figure the amount that is deductible on Schedule C or C-EZ.