A set-up type projectile advances a fixed distance, or starts right in position, and basically stays there for some time (or moves slowly and ominously, allowing you to move independently from it).
Some have an attack hitbox as long as it is active, and others require an activation in order to be active.
The goal is rarely to hit the opponent with the projectile directly, but rather to limit the opponent’s motion, whether to slow it down (e.g., creating a corner-like situation not actually in the corner), to set up a crazy juggle (very commonly exhibited with the Aegis Reflector in SFIII), or simply to keep the opponent afraid because you control the area.