It's a matter for facts and circumstances. If you and I rent a house and split the costs, neither of us is making the other pay us rent with the intent of income. Even if only one of us is on the lease, and makes the rent payment, and the other roommate then reimburses for their share, that isn't renting a room for income purposes. But, an example of my owning or renting a 6-bedroom house near a university, and then charging by the room to students rotating in and out of school, is an example of rental income for me.

The IRS has guidance for this. Examples:

https://www.irs.gov/taxtopics/tc414

https://www.irs.gov/newsroom/know-the-tax-facts-about-renting-out-residential-property