Result cache doubt

Question

what is the difference if i write relies_on clause or if i dont write it, technically please explain what exactly is the difference ??
create or replace function tax (i in varchar2)
return number
result_cache
is
begin
return i*0.08 ;
end;
create or replace function tax (i in varchar2)
return number
result_cache relies_on (emp)
is
begin
return i*0.08 ;
end;

Answer 1

http://docs.oracle.com/cd/B28359_01/appdev.111/b28370/function.htm#i34368
RELIES_ON
Specifies the data sources on which the results of a function depend. For more information, see Using the PL/SQL Function Result Cache. http://docs.oracle.com/cd/B28359_01/appdev.111/b28370/subprograms.htm#BABFHACJ
To enable result-caching for a function, use the RESULT_CACHE clause. When a result-cached function is invoked, the system checks the cache. If the cache contains the result from a previous call to the function with the same parameter values, the system returns the cached result to the invoker and does not reexecute the function body. If the cache does not contain the result, the system executes the function body and adds the result (for these parameter values) to the cache before returning control to the invoker.Not sure why are you talking about invalidated data, result_cache just invoked result from a previous call to the function if it exists, if not it executes the function body.
The clause relies_on just specifies the data sources on which the results of a function depend.
am i missing something??