When a tracker uses linear slides, each octave is divided into an equal number of "micro-notes", and as a result, pitch sliding effects like portamentos and vibratos cover the same range of pitch no matter what octave they're played on. Without linear slides (aka "amiga slides"), only the period of the note is used (the reciprocal of the frequency), and pitch sliding effects change the period directly, so that a vibrato at C-5 will be twice as deep as a vibrato at C-4.
NNA (New Note Action) allows you to specify what happens when a new note occurs in a channel where a note is already playing. Cutting the note is the default behavior, and is what occurs in formats without NNA. Other options are Note Off (which starts the envelope release), Note Fade (to linearly fade out), and Continue (continuing the envelope without release). NNA requires "virtual channels" to support the multiple notes that may be playing in any given channel of the pattern.