Steel channel for cable - what does 12mm, 25mm etc. mean?

I'm going to get some hat section steel channel, but not having used any before can someone tell me what the standard 12mm, 25mm and 38mm dimensions refer to please?
Is it the whole width of the channel, or just the width of the section that carries the cables?
Thanks for any help
Steve
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clangers snipped-for-privacy@yahoo.co.uk (Steve North) wrote in message

width of the section that carries the cables?
YES !

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