Documented targets: minimal, full, extra

Hi,

In one occasion, Peter Dimov pointed out to me that
there are three documented targets with known names:
minimal, full, extra

1. Where can I find documentation of those?

The https://boostorg.github.io/build/ does not seem to mention.
I've failed to dig out anything via b2 --help

2. Are those available via Boost.Build or some Boost-specific configuration?

If the latter, shall I forward my questions to Boost ML?

3. What is recommended usage of those targets?

For example, I'm trying to improve/refine/clean-up GIL Jamfiles
and I don't know if the simple and full defined here
https://github.com/boostorg/gil/blob/develop/io/test/Jamfile
are considered local to GIL or global to Boost?
Is that libraries are supposed to define minimal set of tests
for simple target eg. to keep Boost CI-s more responsive?

What shall I read to get those question answered?

Best regards,
-- 
Mateusz Loskot, http://mateusz.loskot.net
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