This test is done to verify that a product can operate as intended even if it is exposed to a strong radio transmitter. The test limit for immunity is much higher than the test limit for emission, so the fields involved are strong.
Be aware that the RF fields a system may be exposed to can be higher than the test limits required for EMC approval. The test limit for conducted RF fields is 3V/m for household applications. A GSM cell phone transmitting at maximum power will produce this field strength at a distance of 3 meters. If the cell phone is closer, the field strength will be higher.
If the intended use of the system may include operation while someone is using a cell phone nearby, it is therefore a good idea to test the system for higher immunity levels than the minimum levels required. Industrial applications usually require 10V/m or higher.
Digital systems usually do not experience problems with this test, but analog parts of the system may.
As for RF emission, the RF immunity tests are split into two different types: radiated and conducted emission.
The test setup for radiated disturbances is given in IEC 1000-4-3; for conducted disturbances the test setup is given in IEC 1000-4-6.