[boost] [boost.build]: docs for the --layout option

I'v just built boost using 'bjam -sBUILD="release <threading>multi" --layout=system' and I notice that all library names contain an 'mt' tag, which I thought would be suppressed with the --layout=system option. Unfortunately, I'm unable to find documentation for the --layout option, so I can't verify whether I remembered it wrongly. Thus I have two questions: * Where can I find complete documentation for options such as --layout ? (I'm not even sure this is part of bjam, or boost.build, or something entirely different.) * What is the correct use of --layout, and what does it actually do ? Shouldn't 'mt' be suppressed, too ? (Did the actual behavior change recently ?) Thanks, Stefan