I seem to recall being told in the dim distant past that one of the differences between Cat 3 and Cat 4 (other than the obvious damage and repair means) was that the cause of / reason for a Cat 3 could (but not necessarily always) be investigated by a Unit Inquiry (appointed by, and reporting to, the stn cdr) whereas the cause of / reason for a Cat 4 or Cat 5 had to be investigated by a Board of Inquiry (convened by, and reporting to, the AOC). Maybe this is the politics that Lyneham Lad refers to?
Purely out of curiosity I would be grateful if anyone who knows for sure could confirm if this is one of the markers for deciding whether to run a UI or BoI.
W