As above, but a little bit more info.
DSL has a couple of different bearer channels over which it can send data - Fast Path and Interleaved Path. If the SP uses INP then they tend to use the interleave channel which allows for easily configurable parameters for error protection. INP=0 or Interleave=1 indicates that data is being transmitted over the interleaved path, but the data isnt actually being interleaved.
Historically we used to say FAST path or Interleaved Path, but since ADSL2 its actually more correct to use the term error protection or INP which covers both interleave and RS encoding.
Its possible to have a small amount of RS encoding without interleave being applied.. in fact Openreach does this on FTTC upstream by default, which is why we may sometimes see upstream FEC despite interleaving being 'off'.