This item is a must if you want to remotely control flash when you take shots. Function of each part: Mounting foot: Fix on the fixed stand. PC port: Connect the flash as output. USB upgrade interface: Connect the computer for firmware upgrade. Test/ Shutter button: Half press and short press for flash test; long press for remote shutter. Radio transmission confirmation light (LINK): Shows green if it’s connected to master unit or shows red if not. Hot shoe: Install the flash. Battery compartment: Install 2 AA batteries. Power switch: ON / OFF. LCD display: Indicate icon of flash mode, flash output power, ID, channel, grouping, battery, etc.. [Set] Setup button: Choose parameters. [+]&[-] button: Change parameters. Compability： Compatible transmitter(master unit): YN-E3-RT/YN600EX-RT/ST-E3-RT/600EX-RT Parts of the compatible E-TTL flash list: Canon EX(RT) / 580 EXⅡ/ 430 EXⅡ Yongnuo YN568EX(Ⅱ)/ YN565C / YN468C(Ⅱ)/ YN467C(Ⅱ)/ YN465C (The items mentioned above are for references only.) Specifications: Distance: 100m (without interference) Channel: Auto, channel 1-15 Group: 5 groups (A/B/C/D/E) Frequency: 2405-2475MHZ Wireless ID: 0000-9999 USB firmware upgrade: Support Battery: 2 * AA (support 1.2V rechargeable batteries) (not included) Working time: 24 hours Product size: 9 * 5 * 2.5cm / 3.5 * 2 * 1in Product weight: 73g / 2.6oz Package size: 10 * 8 * 6.5cm / 3.9 * 3.1 * 2.5in Package weight: 152g / 5.4oz Note: This item includes receiver, cable and adapter plug only. Other items showed in the picture are not included. Package List: 1 * Receiver 1 * Cable 1 * Adapter Plug