"All PCL Command Sequences begin with the escape character (an ASCII control code with a hex value of 1Bh, 27 decimal) and end with the first upper case letter (or "=" note: "@" is considered an uppercase letter). In between there is either nothing (the PCL command is just the escape and a single upper case letter) or a set of one or two characters and often a numerical value. The single character commands are listed at the end of the "Decoded PCL Commands" table below. The first non-uppercase character after the escape will be a symbol between "!" and "\" (most commonly one of %,&,( or *) which is called the "parameter." After the parameter there is normally a lower case letter called the "group." After this there may be a numeric value. If no value is present, the value is assumed to be zero. Finally the ending uppercase letter is called the "command.""
http://www.sxlist.com/techref/language/pcl/index.htm
