Because libstdc++ based its implementation of the STL subsections of the library on the SGI 3.3 implementation, we inherited their extensions as well.
They are additionally documented in the online documentation, a copy of which is also shipped with the library source code (in .../docs/html/documentation.html). You can also read the documentation on SGI's site, which is still running even though the code is not maintained.
NB that the following notes are pulled from various comments all over the place, so they may seem stilted.
Generated automatically by Doxygen for libstdc++ from the source code.