What's the difference between a disposable and a refillable fountain pen?

Disposables like the Pilot Varsity and EYEYE come pre-filled and are designed to be thrown away when empty — no converter, no bottled ink, no maintenance. Refillables like the Lamy Safari and Hongdian Black Forest accept ink cartridges or a converter for bottled ink, so the up-front cost is higher but per-page cost drops dramatically.