SLA - Shared Line Appearance
Shared Line Appearances: SLAs allow you to place a call on hold at one set and pick it up easily at another set. SLA is also known as SCA: Shared Call Appearance. You can join an existing conversation be pressing the corresponding line button. Typically the phones will have dedicated buttons with LEDs for each of the shared lines.
BLF (busy lamp field)
BLF stands for Busy Lamp Field. It will show you if the user is on the phone or not, and allows you to use that button as a speed dial to call that user. You can not make a call on that user's line from your phone.