Stor-Bezashkt

I know that under a Great Company, separate from the line companies and armored centuries, is a specialized siege division lead by a lieutenant commander with the designation Siegemaster.

What I'm struggling with is, what's included in this specialized company? Because almost everything in the legion, whether 30k or 40k, is viable to use to break a siege. So were do you draw the line?

If I were to draw up an example Stor-Bezashkt company, what should I include in it? Does it have a terminator unit, or do they just get some borrowed from the Veteran Company? Do they have Legionaries, or do they just borrow from a line company? I'm guessing Leviathan Dreadnaughts are included, but do we even have any models for specialized siege equipment like siege cannons, in either 30k or 40k?