Meters used for power measurements at radio or microwave frequencies are generally classified as absorption type (containing inside their own load, generally 50 W for RF work) and transmitted or through line type (where the load is remote from the meter).
Apart from the type, power meters are mainly based on thermistors, thermocouples, diodes, or radiation sensors. Therefore, to work properly, the sensor should sense all the RF power (P-LOAD) incoming into the sensor itself.