A shower valve is architecturally split into two distinct parts: the valve body, which is buried deep inside the wall and cannot be changed or replaced later, and the trim kit, which is visible and can theoretically be replaced. The two parts must be matched and compatible at the point of purchase. A particular valve body will only work with certain compatible trim kits; they are sold together as a matched system. Choosing the valve body and trim kit is a single integrated decision, and trying to change one without the other later is not practically feasible and may be impossible.
The depth the valve body is set to during rough-in is permanent and locked in. The body sits in the wall at a specific calculated depth that accommodates all of the finished wall layers — the tile surface, the thin-set adhesive, the waterproof assembly, all the protective layers between the structural wall and what you see and touch. That depth is calculated and planned before the wall is built and the rough-in happens. If the wall build-up composition or thickness changes after rough-in, the trim escutcheon will not sit flush against the tile, or worse, it will not sit or fit at all. The valve depth cannot be adjusted or modified later without opening the wall and moving the entire valve body.
A shower diverter is a valve component that routes water flow to different outlets — a fixed showerhead, a handheld shower, or both simultaneously. When more than one outlet could be active at once, the diverter decides where and how water is distributed. The specific diverter arrangement and type chosen at the time of valve purchase determines what outlets can be active at the same time and what the physical control lever positions actually do and mean. That diverter arrangement is locked in permanently when the valve body is chosen and installed. Changing it later means replacing the entire valve body, not just the trim.