When sending type 3 messages, i.e. a message with a six character locator e.g. JO65FR, in WSPR the call sign is hashed. This means that instead of sending the call sign a 15 bits hash code is sent. But the hash code is not unique to the specific call sign but “shared” with some other call signs. In such cases hash collision can happen if both transmitting stations are decoded by other stations irrespective of only one of the transmitting stations is sending type 3 messages.
Both call signs must have 3-10 characters and they must be ITU compliant. Add-on prefixes can be up to three alphanumeric characters, add-on suffixes can be a single letter or one or two digits. WSPR permits either an add-on prefix or an add-on suffix but both, i.e. calls like OZ0RF (3245), OZ0RF/P (26800), OH0/OZ0RF (25748) are all OK but OH0/OZ0RF/P is not.