Do I just use TotalBoat Halcyon Satin varnish as a final coat, or can I build coats with it, too?

TotalBoat Halcyon Satin should only be applied as a final coat, ideally. Halcyon Clear Gloss or Amber Gloss should be used for the build coats.  

As a general rule, any varnish material should be built up using a glossy material, then, if a satin or matte finish is desired, only apply the satin or matte as the final layer. This will give the best image of the grain, and richest colors.

For best results, apply all build coats (at least 3, but applying more will increase the depth of the image), allow to cure for 24-48 hours, sand lightly in the direction of the grain with 320-grit sandpaper, remove all sanding residue, and apply Halcyon Satin, as directed.