sapphire radeon r9 280x 3gb has a small button

Hi OP,

It is a switch that can be flipped at either sides. It means your card has DUAL BIOS. So, you can normally call the button as "Dual BIOS Switch". One side is to use the card with base clocks and the other side is made especially to boost the performance by overclocking the card to specific clock values (both graphics and memory).

Also, if one BIOS gets messed up, you can use the other BIOS and continue with the same graphics card. Hence, this helps you to save your money.

Edit: I hope you meant this switch as the button. Peer into the center part of the image and you can see a tiny black switch.
amd-radeon-6990-bios.jpg