This all comes down to preference on what kind of finish you want your walls or ceilings to have. There are many options of sheens, and can get confusing. Let's break them all down.

Flat

This sheen is non reflective. This is more modern look in homes right now. This is often used in low traffic areas, textured walls or ceilings. This is a durable finish and is easy to clean.

Satin

More reflective than a flat finish. Provides some sunlight without being over reflective. Great for medium traffic areas. This finish is easier to clean than flat. This finish is similar to an Eg-Shel.

Eg-Shel

Similar to satin finish. Around the same light reflection with a velvet looking finish. The general information applies, good for medium traffic areas. Easier to clean than a flat sheen.

Matte

Again, similar to both Eg-Shel and Satin sheen. Low light reflection. Good for medium traffic areas and is easier to clean than a flat finish.

Semi-Gloss

The most light reflective finish listed thus far. This provides a good reflection of light to naturally light up the room. Great for high traffic areas and is very easy to clean. Often needs multiple coats of paint.

Gloss

The most light reflective sheen. Provides a very shiny and reflective surface. Great for high traffic areas and is extremely easy to clean. Just like semi-gloss, often needs multiple coats of paint to achieve the best results.