This can be expressed in UV transmittance (UVT) or intensity. In many cases, this information is not public domain and you must request the information to be included in the technical specifications of the proposal. Systems with lower UVT limits (down to 40%) and lowest intensity readings are those which provide you with the most flexible operation.
Water type and water temperature
These are additional limiting factors for the envelope of operation, make sure the certification includes all or as many as possible types of water (sea, fresh, brackish) and that water temperature is not a limiting factor (if using electro-chlorination). For example, electro-chlorination BWMS cannot operate in fresh and brackish water. This is a considerable limiting factor on the vessels ballasting and deballasting operation.