Relationship expectations
Clarify whether both people see this as casual, whether either person is actively dating, and whether a conventional romantic relationship is currently off the table or simply not assumed.
Exclusivity
Ask rather than infer. If either person wants sexual exclusivity, define what that means and what happens if circumstances change.
Sexual health
Discuss protection, testing, contraception where relevant, other partners and what health information needs to be shared. If you cannot comfortably have this conversation, that is useful information about readiness.
Communication and frequency
Some people want friendly everyday contact; others prefer occasional messages tied to plans. Neither is inherently more casual. The problem is assuming your preference is shared.
Privacy and social life
Talk about whether friends know, whether public affection is comfortable, and how you will handle mutual social circles. Privacy should be mutual, not a unilateral demand.
Changing or ending the arrangement
Agree that either person can pause, redefine or end the relationship. You cannot pre-negotiate every emotion, but you can establish that changes will be discussed rather than punished.
A practical pre-start check
Ask yourself three things before raising the idea. First, would you genuinely accept a no without trying to wear the person down? Second, are you being honest about wanting something casual rather than quietly treating FWB as a stepping stone to commitment? Third, are you prepared to discuss sexual health and boundaries as directly as you discuss attraction? If the answer to any of those is no, slowing down is more useful than finding a cleverer line.
Clarity should make opting out easy
A strong beginning is not measured by whether the other person agrees. It is measured by whether they understand what is being proposed and can choose freely. That makes the arrangement more informative for both people: a yes means more, and a no can be respected without turning the friendship into a negotiation.
Labels help describe a relationship, but expectations are specific to the people in it. When something important changes, a direct conversation is usually more useful than trying to decode a rule from the internet.