The ε-TeX extensions to TeX are a range of new functionalities added by the NTS to Knuth's original TeX program: see details of engine developments for more.
In a modern TeX distribution, the extensions are enabled for almost all formats
except Knuth's TeX (
tex) itself. As such, most users will never use a
system not featuring ε-TeX: indeed, LaTeX2e has required them since