Panel Meter is Auxiliary Device which shows the electric value on panel board, ex: voltage, current, frequency.

Current Transformer (CT) is an instrument transformer in panel board. According to Electromagnetic induction, high current througth the CT's window (Primary Current) can induct and provide 5A / 1A (Secondary Current) on CT's connector for measurement.

CT's Secondary Current connector can’t be kept open, otherwise secondary current is 0A and cause high voltage, which burn the CT secondary winding and can damage the system.Therefore a load on CT's Secondary connector is necessary and it is called VA (Burden).

AC means Alternative Current. Voltage and current periodically reverse direction and change the magnitude.
DC means Direct Current. Voltage and current fix the direction and magnitude.
 

UL stands for Underwriter's Laboratories, which certifies products, facilities, processes or systems based on industry-wide standards. It helps customers to purchase evaluated products and prevent from electrical damage.