>> There are a bunch of LaTeX packages that only work with a specific
>> TeX engine, and which need special input code for that. For
>> example, `fontspec` (with its excellent OpenType support) only
>> works with XeTeX and luatex. Or think of 'lyluatex', which
>> obviously needs luatex.
>
> Yes, absolutely. This is exactly why I am surprised that some people
> set global environment variables that select a TeX engine "to always
> use the same". What tools do they have an effect on?
I forgot to mention that *all* TeX flavours understand 'normal' TeX
and LaTeX code that was written for the original TeX incarnation. If
you only work with such code and don't have to or don't want to deal
with extensions like OpenType font handling, it often makes sense to
replace `pdftex` with `xetex` or `luatex` since the latter two
programs usually produce *much* smaller PDF files.
Werner
