What is ε-TeX?
The 
-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](FAQ-enginedev) 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 -TeX: indeed, LaTeX2e has required them since
2017.
Sources:
