Sheen is how much light a dried paint film bounces back. The more it bounces, the tougher and more washable the surface is, and the more ruthlessly it reveals every dent, seam, and patch underneath. That single trade-off explains almost every sheen decision in a house.
Flat and matte belong on ceilings and on walls in low-traffic rooms, especially older walls with a lot of history in them. Flat hides an uneven surface better than anything else, which is why it is the right call on a plaster ceiling. The cost is washability: scrub a mark on a flat wall and you will often burnish a shinier spot into it.
Eggshell and satin are the working finishes for most living space. They take a wipe without burnishing, hold up in hallways and bedrooms, and still hide a reasonable amount of wall imperfection. If someone asks what to put on the walls of a normal house, this is the answer nine times out of ten.
Semi-gloss and gloss belong on trim, doors, cabinets, and the wet walls in bathrooms and kitchens. They are hard, they clean up, and they take being knocked. They also show every ripple, which is exactly why trim gets sanded and filled properly first. A semi-gloss door with a lazy prep underneath looks worse than a flat one.
One practical note that matters more than the chart: sheen only performs if the prep matches it. Higher sheen demands a flatter, cleaner, better-primed substrate. If a painter suggests dropping to a lower sheen to hide a wall, ask whether the honest fix is a skim coat instead. That conversation belongs in the estimate, not on the day of.