All examples contain actual data as of when each section was originally written or was last modified. P 'P'rime, perhaps with info about the discovery. See primeM.txt for a complete list of known Mersenne prime exponents. The number of decimal digits is recorded since that will change every time a new factor is found. Mersenne is known to be composite.
Extent is the highest trial factor attempted.
The extent may be in power of two format as explained for 'H' lines. Same as 'U' except that extent itself should be re-checked. Mostly obsolete as of early 1996 as the only factorer that prints it is obsolete except for checking known factors. The largest prime factor may not be listed explicitly in a 'C' line. See also the 'L' and 'M' lines below.