The feature is called "Cell Size Reduction" and it makes the access point "deaf" to devices beyond a certain energy level. The main purpose is if you have a very, very crowded area with alot of access points and you want access points to only service clients that are very near to them. You would require much greater access point density, because you would have to reduce the power on the access point to match the power on the clients for the best performance. Of course many different clients have different transmit power, so contacting support to see if you even need this or what parameters you should use to service your clients in the best way possible is mandatory.
The vast majority of deployments, even dense deployments, require extensive testing with this depending on the type and number of different clients you have to determine if it should be used, or not.