It seems natural that each line would be evaluated left to right then the next line down etc-but it doesn't work this way. For each (6 line) rung, ALL the inputs are read, ALL the logic is figured out, then ALL the outputs are set, then the next rung is evaluated etc. This can presents a problem in certain circumstance if the output of one line feeds the input of another
Classicladder is the original work of Marc Le Douarain.
The most up-to-date information is in the user manual, search for "classicladder": http://linuxcnc.org/docs/html/
Classicladder ver 7.100 (EMC 2.1 - 2.2.8) While specific to version 7.100 much of the info is relevant to later versions of CL.