Niches and benches are their own lines
A niche, a bench, or a window return adds corners, a shelf that has to shed water, and waterproofing folds that a flat shower wall does not have. Listing "tile the walls" does not describe those pieces. Each one belongs on its own line, with its size and with a count of the faces that get tile.
Count the faces
A rectangular niche has a back, a sill, and two jambs, and sometimes a head. A bench has a top and a front, and often two ends. A window return has jambs and a sill that may already exist in another material. Write the count. The word niche can mean a manufactured box or a framed opening built in the wall, and those are different amounts of tile and different folds in the waterproofing.
The shelf has to shed water
The scope should say that the sill or the bench top falls so water leaves the horizontal surface, and that the waterproofing is continuous through the corners of that shelf. A flat shelf tiled like a picture frame holds water. The fall is a requirement on that piece. It is not implied by the wall tile around it.
Size is part of the line
Give the width, height, and depth you want, or say that the size will match an existing opening you have measured. An unmeasured shelf becomes whatever depth the studs allow, which may not fit what you meant to store. If two niches are wanted, write two lines. A plural hidden inside a vague details phrase is how the second one disappears.
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