I take it your refering to the Field limit take off mass graph on page 55 of the CAP manual.
If you are, then this graph ends up giving you a weight. This is the weight, above which you will not be able to comply with the take off regulations. What I mean by that is:
1) either make it out of the airfield because you might over run the runway before getting airbourne.
2) or in the event of an emergency stop, not be able too before the end of the runway.
It's a lot more complicated than this, but essentially that weight that the graph gives you, is taking into account the ASDA and TODA.
Look at the bottom of the graph, an you see it say "Field length available". Well, now look page 53 at the top is says that this is in fact a balanced field. Balanced fields are when the TODA and ASDA are the same. But in the performance manual it assumes it is also the TORA.
If you get confussed ingnore the last few lines of this post.
Steve