Hi Andy Neil,
Thanks for your views. The reason I am starting this thread is:

About 3 years back I had developed the software for one of my clients on MCS51. Now he wants to develop the same product (with added features) using ARM Processor. I do not have expertise on ARM, so he is getting it developed form some other developer. This new developer wants to understand the MCS51 based product, and then build it using ARM processor. All is OK till here.

The problem is: He asked for some documents related to software, that I have never heard of..and I don't even remember their names now. Anyways, I explained the whole project to him for a few days, and then the great light dawned upon him. Now he is on his own, of course I am there to help him out.

But the big question is: I had to waste my (current) precious time after 3 years just to explain the whole project all over to him. So I was thinking, is there an ideal way of documentiion? Such that my current time is not wasted. I am sure many other members must have also faced this problem.