On Sun, Nov 3, 2019 at 12:46 PM ERR ORR via Boost-build <boost-build@lists.boost.org> wrote:

I am writing for help building V1.71 and additional modules Asio,
Checking the log output, the build tool appears to not build these
modules by default.

I think it would be helpful it you provide a detailed issue report. For example, the guidelines at


are very good. 

  - Could someone please explain to me how I build exactly ONE
additional module, for example Asio?
- Could somebody please explain to me how I can build the whole
library, module by module, dependably, to <stage>, if must be,
and from there to the <INSTALL> destination?

As documented in the getting started document, you can do that with

b2 --build-type=complete stage

If you wish to build just one module, you can use the "--with-xxx" option.
- Could somebody please explain to me why Boost has decided to NOT
include certain modules in default build process, and how do I circumvent this?

To answer you question, it's necessary to work from a complete issue report, as I suggested above. Some modules might require additional dependencies, and
are not built unless those dependencies are detected. The decision is usually made by the module author, and without any output, it's hard to guess what's
going on.
Secondly, the build tool appears to not be documented correctly.

Sadly, this is not an actionable issue report either. If you create an issue explaining what part of what documentation you find incorrect, it might get fixed. 
Vladimir Prus