I am not sure of how best to write it but it means a construction that maintains waterproof when being cleaned. I worked in the automotive industry for 6 years and one of the requirements for the engine compartment was being able to withstand 1 minute of spray from a pressure washer and this is much, much more stringent than rain or snow getting into the engine compartment. That being said, I don't know what it would have to do with a software requirement.