Purchasing decal in the Elite Dangerous online store allows you to use it in-game via any Starport station services.
You can use any combination of decals, or use the same decal more than once, provided your ship has multiple decal slots. The paint job currently in use on your ship will affect the look of decals you choose to apply. When docked in your ship, bring up the Starport Services Menu. Navigate to the Outfitting menu and select 'Livery'. Any decals you have purchased will be available to be applied to your ship.
• These decals do NOT alter the performance parameters of your ship in any way. They are purely cosmetic items.
• If you replace your ship you will need to reapply any Decals you wish to use while in the Livery at the Outfitting menu.