interest tracing rule

用在自住房上，需要通过home mortgage interest deduction

用在出租房上，可以抵消这个出租房的出租收入。

如果用在度假，大学学费，医疗费，律师费，不能抵消任何利息。

When a taxpayer borrows funds, the deductibility of the related interest expense is determined by how the loan proceeds are ultimately used and not how the loan proceeds are collateralized.

https://www.pkfod.com/insights/federal-interest-tracing-rules?