They are all the same thing.
RSSI, Amplitude, mW and dBm all measure the same thing.
Problem is though is none of them are calibrated, and RSSI is calculcated differently by different vendors.
So you should not compare RSSI values between different devices. However, if you stand at point A and measure with a device, then move to point B and measure with the same device, you can compare the differences and get valuable information.
They are the same thing however we always use RSSI in wireless networks and is more specific to the received signal. The amplitude could also mean the strength of the transmitted signal, RSSI is the received strength at any point. RSSI is mostly measured in dbm, Amplitude is measured in watts/milliwatts in most cases.