UPC 804.1 'Standpipe Receptors' reads 'No standpipe receptor for a clothes washer shall extend more than 30 inches (762 mm), or not less than 18 inches (457 mm) above its trap. No trap for a clothes washer standpipe receptor shall be installed below the floor, but shall be roughed in not less than 6 inches (152 mm) and not more than 18 inches (457 mm) above the floor.'

Measured to where? the bottom of the bend? the top of the inlet? the middle of the outlet?

It depends on if your stand pipe is exposed or inside the wall. If inside the wall a 45 degree elbow must be used to expose the opening.

I use the weir of the trap(bottom of the outlet) to determine the 6" dimension.

as a general rule, rough in dimentions are given from the center of the pipe.
unless otherwise specified

I don't understand your answer. I have an outlet box for the supply lines and the drain. The pipe will be inside a wall. Where would the 45° bend go?

To @steve: thanks. That makes sense. But the code reads, to me, as though both rules refer to the same thing. Do you measure the distance above the trap from the bottom? I plan on keeping the bottom above 6 inches, can vary the trap's location enough to make the distance above fit either interpretation.

2022 California Plumbing Code

804.1 Standpipe Receptors
Plumbing fixtures or other receptors receiving the discharge of indirect waste pipes shall be approved for the use proposed and shall be of such shape and capacity as to prevent splashing or flooding and shall be located where they are readily accessible for inspection and cleaning. No standpipe receptor for a clothes washer shall extend more than 30 inches (762 mm), or not less than 18 inches (457 mm) above its trap weir. No trap for a clothes washer standpipe receptor shall be installed below the floor, but shall be roughed in not less than 6 inches (152 mm) and not more than 18 inches (457 mm) above the floor. No indirect waste receptor shall be installed in a toilet room, closet, cupboard, or storeroom, or in a portion of a building not in general use by the occupants thereof; except standpipes for clothes washers shall be permitted to be installed in toilet and bathroom areas where the clothes washer is installed in the same room.

If you have a box inside the wall with waste and water, of course you don't need a 45.

Thanks. California's is a little bit different. You say weir is the bottom of the trap, that the 18-30" means that the top of the pipe in the outlet box can't be more than 30" above the bottom of the trap?