What does this variable do?

[G_yoshi]

I was wondering what the last variable in the addtiledef command really does. I noticed that the newfeatures2001.txt states that it does something to the tileset when you change it to 1 instead of 0....it also said that what it does will be revealed later on.....well, I am asking what it is that it does.

Its just kinda been on my mind as to what its used for and how it works.