This has been covered a few times on the forum already.
Before April, 2017, walls height was "as constructed" unless otherwise stated in the WSB. Ie, you could shoot over them if you could see over them.
However, USPSA published a "ruling" last year (this is different than an interpretation). Said ruling is available on the USPSA site under "rulings".
Ruling:2.2.3.3 is amended to read: Unless otherwise specified in the written stage briefing, all such barriers, walls, vision barriers, snow fence barriers and other constructs will be considered to go from the ground to infinity, provided said barrier is at least 6 feet, (allowable variation = -3 inches) in height. Any barrier less than 6 feet tall (-3 inch variation) specified as extending to infinity must be clearly identified in the WSB and marked accordingly, otherwise, all barriers less than 6 feet tall are considered to be \"height as built\" and may be used accordingly by competitors.