e-TeX intends to fill the gap between TeX3 and the future NTS (New
Typesetting System).  It consists of a series of features extending
the capabilities of TeX3.

Since compatibility between e-TeX and TeX3 has been a main concern,
e-TeX has two modes of operation.  In TeX compatibility mode it fully
deserves the name TeX and there are neither extended features nor
additional primitive commands. That means in particular that e-TeX
passes the TRIP test without any restriction.  There are, however, a
few minor modifications that would be legitimate in any implementation
of TeX.  In extended mode there are additional primitive commands and
the extended features of e-TeX are available.