A dub is an event that consists of one or more tracks being muted or unmuted at a particular time. Dubs are useful for arranging tracks into a song, which is essentially a sequence of dubs. You can create dubs by drawing them in song view, but it's often easier to create them by recording an arrangement that you perform in live view.
Dubs don't support fading. To fade a track in or out, either create a velocity modulation that targets the track, or create a controller track that changes the instrument's volume (typically controller number seven). These fader tracks can also be dubbed in and out, like any other track. Another option is to use a MIDI control surface to change instrument volumes live, and capture the control changes by recording MIDI input. If you're exporting your sequence to another music software, you may prefer to create fades within that software, as it may have better automation support.