>% Philip Taylor writes:
>%  > >>   L:  works under LaTeX
>%  > >>   OL: works under LaTeX 2.09
>%  > >>   P:  works under plain TeX
>%  > >>   X:  support application, OS specific
>%  >
>%  > +:=  E1: requires e-TeX V1
>%  >      E2: requires e-TeX V2
>%  >      On: requires Omega Vn...
>%
>% shall we just let the authors provide this clue? rather than trying to
>% test it independently?
>
>d'accord.  ** Phil.

  I think the normal thing would be that an author develops package for a
particular TeX format, but then again, such a package may well run under
another format, and the author may not have the particular
hardware/software needed to check it out. So why not
  \DesigedForTeXFormat{Foo}  % Supported by developer
  \RunsUnderTeXFormat{Bar}   % Unsupported, but seems to run
"Unsupported" means that it runs under a particular TeX format, but an
upgrade of the package may make that upgrade to not run any longer under
that format.

  I thionk also that a more advanced package may want to choose its code
depending on which TeX format it is run under, so one may want to have a
command
  \IfTeXFormat{Foo}%
   {Do this stuff}%
   {Else do this stuff}
One can then conditionally include the files needed under several different
TeX formats.

  Hans Aberg