Answer:
substitute products.
Explanation:
A substitute product is one that can be used for the same purpose as another one. When a customer demands more of one of the products, demand for the other one will fall. That is they have negative correlation.
For example if accounting software is used instead of paying a CPA to do your taxes, the accounting software is acting as a substitute for the services of the CPA. The more the customer uses the accounting software the less he will need to engage the CPA for his tax preparation.