This looks as if it defines it pretty well. http://doctorsforadults.com/journals/news/jun01/q&a.htm

All you need is a statement from your doctor stating that you qualify as a homebound patient. Explain briefly why you are homebound in the letter. Be sure to get a copy. If you draft the letter for him, perhaps he will be better able to get it done in a reasonable time. Most doctor's nurses do this for the doctor they work for and if you are at all friendly to her, you might do this for her. I know my doctor's nurse is so rushed that she would be glad to have me hand her a drafted letter for signature.

Billye