An omni-directional antenna sends the signal in all directions (like a sphere around the beacon) where as a directional antenna focuses the signal primarily in one direction.
Our Series 21 beacon has two physical antennas inside which can be individually selected by way of how you configure the beacon. By default, the omni-directional antenna is selected and is suggested for the far majority of use cases.
If you reconfigure the beacon to use the directional antenna, note that the signal radiates from the front of the beacon (with the button and LED) like a spotlight that is approximately 120 degrees wide. Also note that it's impossible to prevent the signal from emanating from the back side of the beacon completely in this mode however it is about 20 dBm less than the front which is suitable for recognizing direction with our SDK at short distances. We recommend only using the directional antenna feature in use cases at very short distances (less than 10ft) where accuracy is important.