A unique index for a rule in a particular role.
The rule are applied according to their rule
number, i.e. Rule 1 will be the first rule applied
followed by Rule 2 and so on.
Rule numbers need not be contiguous, for e.g. a Role
can have three rule numbered 1, 4 & 7. Further when
a new rule is added to this Role it can be rule number
2 or 5 or 9 (any number other than 1, 4 and 7).