...a BIT thick? That's an understatement, and I'm totally thick as I can't interpret your question. It's usually 2 brackets to the case. Doh! can you rewrite the question please?
Ah! I've just worked it out, they are dimensions to be observed if you have difficult site conditions which are difficult to equate to those of BS5410 (if you're not sure give it loads of space).
Then again, you could do what some building control officers do and ignore it if it's awkward.
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