Per @TaxGuyBill in this thread : https://ttlc.intuit.com/community/investments-and-rental-properties/discussion/use-heloc-from-my-pri...
By default, it is Home Equity Debt (which is not deductible for 2018). There is an "election" to make it not Home Equity Debt and to 'trace' the amount for what it is used for. That "election" is made by just entering the deduction for what it was used for (such as a rental). See §1.163-10T(o) for the legal gibberish.
