The portfolio acquisition model is essentially a decision-making tool used to produce a bid that investors will make on an existing portfolio of loans. In this case, the loans are Home Equity Lines Of Credit (HELOC) (or ‘second mortgage’ because they are secured by the borrowers property, as is the ‘first mortgage’) typically with a maximum term of 15 years, with interest-only payments terminating in a final bullet payment. Borrowing is allowed up to the credit limit at any time at a variable rate tied to prime–can be fixed or floating–partial principal repayments are allowed, but not scheduled.

https://osf.io/preprints/inarxiv/9765p/download