My dad always taught me that if you are using emulsion on the walls or ceiling you should apply gloss afterwards to avoid splattering from the emulsion. However, if you are wallpapering, you should apply the gloss first. I have followed this advice and never gone far wrong!
I am no expert so I am open to correction here ! Emulsion and gloss are both finishing/overcoat paints. If you want an undercoat you should use a primer/sealer. Emulsion is available in matt (non shine) and silk (shiny) and you could get away with using a matt emulsion as an undercoat on walls and ceilings. Gloss should never be used as an undercoat because it dries with a shiny finish that won't allow the next coat to bond well. On wood I would advise using a primer regardless of what finish you want.