An interim CMO is typically full-time but temporary — filling a leadership gap while a company searches for a permanent hire, often after someone leaves suddenly. A fractional CMO is part-time by design and can run indefinitely; it's not a placeholder for a future full-time role, it's a standing operating model some companies choose to keep.
The two get confused because both are non-permanent, but the intent is different: one is a bridge, the other is a deliberate structure.