What is sedentary or light work?

Whether in workers comp or Social Security disability we often come across the term sedentary or light duty. For example it is common for someone with a back injury to be at light duty levels or have even more restrictions so he is at sedentary levels. The importance (in a legal sense) to this is that it can relate to how employable a person really is and the answer to this can greatly affect his or her benefits. Especially in Social Security cases age and restrictions play a major role in qualifying for benefits. In workers compensation cases they are also significant although it is more complicated since they play a role with temporary and permanent benefits but impairment ratings may also be significant. Strictly from a safety standpoint I've always felt that obtaining restrictions lets you and possible employers know what is the extent of safe phyiscal activity. In any event here is a link to a "disability doc" who sets forth the accepted definitions of sedentary, light, medium and beyond levels of activities:
http://www.disabilitydoc.com/work-descriptions/