Extruder Hold - it keeps the extruder motor from being disabled by command "M103" and is of use for 3MM extruders - source - http://www.makerbot.com/sailfish/releases
Check SD Reads - sounds like it does an error check against what was received - http://jettyfirmware.yolasite.com/v73-v43.php#crc
P-Stop - looks to be so you can add an external trigger to stop the printer via hardware - http://jettyfirmware.yolasite.com/v74-v44.php#pstop