Typically there is also a requirement from the building and fire codes to provide fire sprinklers above the finished area, when they are not fully demised. They look at it from a different perspective, sprinkler contractors often say we are protecting the floor area but if the area above the ceiling is not fully demised from the finished area below (an example is a warehouse with a finished office space along a wall) then you must protect the area above the ceiling as well as the finished space below the ceiling. A cloud is similar, if it is not demised the "open plenum" space above the cloud must be sprinklered too. I don't recall what edition the verbiage was inserted into the building code, but two topics that were related were fires flashing up into the ceiling plenum due to open areas of the ceiling and the warehouse side protection came about as people were storing cardboard and other items on their lay-in ceilings and fire marshals were tired of telling owners / tenants off when they said it was not a code requirement to demise the space.
BTW- a solution is to run a vertical soffit from the cloud to the deck and paint it black. I know just what the other designer is talking about though, I did an office area in a 42' clear warehouse, they put in three levels of slanted clouds, none of them intersected each other but 90% of the floor area was obstructed from protection at the roof deck level. We installed hard pipe drops, some went through a higher cloud to get to a lower cloud, and in the end all the clouds had fire sprinkler protection below them.
