When do I call a service fault, and when do I call a replay?

Call a fault when you're sure the serve broke a requirement (Rule 20.E.1), for example:

• Volley serve: arm not moving in an upward arc, highest point of the paddle head above the wrist, or ball contacted above the waist
• Release not from only one hand or only the paddle
• Spin or manipulation added on the release
• Drop serve propelled in any direction, or released from an aided height

Call a replay (before the return of serve) when you aren't certain a requirement was met (Rule 20.E.2), for example a questionable release visibility, foot position, upward arc, paddle-head height or ball height.

You must call a replay when you're certain the release wasn't visible, and whenever it's questionable whether a drop serve was propelled.