They may have different meanings, depending on whether they are sent as a question or an answer. For example, the message "QRP" means "Shall I decrease transmitter power?", and a reply of "QRP" means "Yes, decrease your transmitter power". They may also be qualified by the word "no", to negate their normal meaning, or the letter "c" to reply in the affirmative. For example, a reply of "QRPC" would mean "Yes, please decrease your transmitter power".
External links:
wikipedia.org dumped 2003-03-17 with terodump