Meaning of gcc-mingw toolset

In the multiprecision testing jamfile there is a line which has the 
requirement:

<toolset>gcc-mingw:<cxxflags>-Wa,-mbig-obj

I do not see a toolset called gcc-mingw nor do I see a subfeature of the 
gcc toolset called mingw. I do realize that the gcc toolset has support 
for mingw gcc on Windows, which I use myself in my user-config.jam with 
the plain designation of:

using gcc : nnn : etc ;

but the nnn is always a version number, like 9.3. The gcc.jam picks up 
that this is mingw-w64/gcc implementation internally and works fine with 
it. But whatever '<toolset>gcc-mingw' is supposed to mean it does not 
applu to my "toolset=gcc-nnn' when I run the test. Can anyone enlighten 
me as to what is going on here ?

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