QCWA stands for the Quarter Century Wireless Association. One can become a member if you have been licensed for 25 years. Your membership number is your QCWA number. Repeats are not valid unless the station worked was portable or mobile in a different state. On 2x1 calls, use the letter, or the district number and the letter in the suffix.
Some countries have more than one prefix.
You can only use a country once. Some prefixes are not usable, for example 3D2, T2 and V6. Use either ARRL DXCC or the one in 1990 or later callbook. Send a copy of the ones you use with your application for the endorsement. Also once you use one of the lists, stick with it and do not change.