A requirement that cannot be met until a prerequisite requirement is met, however, the prerequisite cannot be obtained until the original requirement is met.
In the original it referred to USAF personnel during WW II. To get out of further flying duty, the airman would have to be mentally unfit. However if one claimed to be unfit, then this was proof that there was nothing wrong with him mentally.
From "Catch-22" by Joseph Heller: "There was only one catch and that was Catch-22, which specified that a concern for one's safety in the face of dangers that were real and immediate was the process of a rational mind. Orr was crazy and could be grounded. All he had to do was ask; and as soon as he did, he would no longer be crazy and would have to fly more missions. Orr would be crazy to fly more missions and sane if he didn't, but if he were sane he had to fly them. If he flew them he was crazy and didn't have to; but if he didn't want to he was sane and had to. Yossarian was moved very deeply by the absolute simplicity of this clause of Catch-22 and let out a respectful whistle."
Because Joseph Heller is taking a poke at Army talk where everything has to have an acronym, a code name or a serial number. Everything is regulated and designated and made to sound important by changing it's name. All specialized groups love their jargon. As to why it has to be 'twenty-two' particularly: source:http://cboek12.org/cresskill/School... Kathryn/AP Lit/Catch-22/Why Not Catch 21.pdf