problem with FilePermission for context

[Jochen Schurich]

is it possible to set a FilePermission in the catalina.policy file that 
every context can access his own directory for read & write ?

i found this somewhere in the internet:
grant codeBase "file:${doc.root}/-" {
permission java.io.FilePermission "${doc.root}/-", "read,write,delete";
};
but i can't find anything about the variable doc.root !?!?
does doc.root means: the context's own directory? - or do i have to set 
an environment variable?

thanx,
joe