how know new surface from which original surface?

zhengyou

I get a 3d MODEL which is consisted by several surfaces(called the original surface). when I divide this model into several parts by a series of parallel planes, each part has its new surface(called the new surface). the question is :

the new face is belonged to a certain the  original surface, but they have different index numbers.

so when I use RS 2008 to get the total area of a certain part,  RS wiil give me a cetain area value with an index number.  how can I know this new index number is belonged to which original surface? in other words, how can I know the new surface is belongde to which original surface? thank you!