What is Cell Efficiency Ration in Exadata?

If you have observed, when we invoke CELLCLI utility, it use to display Cell Efficiency Ratio in banner. Example given below.

So what does cell efficiency ratio indicate?

It is the ratio between data accessed by the storage cell and data processed by the database for the cell. Higher the value greater the I/O savings as a result of offloading.

The Cell Efficiency Ratio is same as the value displayed for offloadEfficiency in CELLCLI>LIST CELL DETAIL as below

Then the ratio of the total MB accessed on the cell to the MB accessed on the host (for the cell) is computed every minute that CELLSRV is active by Management Server (MS). 

It is difficult to explain as there is no manual intervention we can do nor we can fetch detailed result of it. We can use it for reference purpose only as there is no practical value from performance perspective of Exadata.