examine the TeX programming environment
Detect “draft” and “final” class options
Provides \ifetex switch
Expandable checks for the current language
Provides the \ifluatex switch
Provides the \ifpdf conditional
Conditionals to test which platform is being used
Am I running under pdfTeX, XeTeX or LuaTeX?
Detects use of VTeX and its facilities
Am I running under XeTeX?