What is the difference between mem_reserved and mem_usage in impala charts?

Hi, 

I am trying to create a dashboard  in CM with impala charts in order to see the maximum memory of impala and the one that my queries are using.

I running tsquery for 

total_impala_admission_controller_local_backend_mem_reserved_across_impala_daemon_pools

and

total_impala_admission_controller_local_backend_mem_usage_across_impala_daemon_pools

But it seems that the charts are representing exactly the same thing. Why is this happens?