If an observer wants HE and then WP what term would they use in the call for fire?

In the context of a call for fire, the term used to indicate that one type of ammunition is to follow another is "followed by." This term specifically instructs artillery units to first fire a specific type of round, such as High Explosive (HE), and then to follow up with another type, such as White Phosphorus (WP). It effectively communicates the sequence in which the munitions should be delivered to the target.

The other terms provided, such as "ammunition" or "projectile," do not convey the necessary sequence of firing specific types of rounds. "Volume of fire" refers to the quantity of rounds fired over a specific time period or the intensity of fire, but it does not address the order in which different types of rounds are required. Thus, "followed by" is the precise terminology needed for indicating a sequence of ammunition types in artillery communication.