As you mentioned, most countries differentiate for indoor- and outdoor use on the 5 GHz band in their regulations. Devices operated outdoors (regardless if they're meant to be used outdoor or just happen to be outside on a sunny afternoon) must adhere to the lower transmit powers allowed outdoors, I would expect that the RBS50Y as a dedicated outdoor devices is limited to those requirements.
Large parts of the 5 GHz band is shared with weather- and military radars, who are the primary users of these frequencies. For indoor devices, the regulatory body takes the attenuation of your outer walls into account and therefore allows higher transmit powers to be used indoors. This needs to be taken into account for devices that are operated outside, without this 'natural' attenuation taking place.
If you do actually mount an RBS50Y outside, I'd expect it to perform better than the overspill from an RBS50 set up inside and having to pass its signals through your external walls.
