What is a under the counter air vent ? How is it installed ? What is the use of this item ?

Are you asking about an air admittance valve (AAV) ? If so, install on the horizontal lateral line between the wall and the trap. An AAV is not going to work for decades so it must be accessible. All fixture need a trap and traps need a vent to allow air to enter the system otherwise water would not drain properly. To see how water drains without a trap turn a pop bottle upside down and watch how it chugs

Are you asking about an air admittance valve (AAV) ? If so, install on the horizontal lateral line between the wall and the trap. An AAV is not going to work for decades so it must be accessible. All fixture need a trap and traps need a vent to allow air to enter the system otherwise water would not drain properly. To see how water drains without a trap turn a pop bottle upside down and watch how it chugs

Thank you for your answer on this matter it will be put to good use THANKS

What is a under the counter air vent ? How is it installed ? What is the use of this item ?

A under the counter air vent or AAV/ Air admittance Vent(see image) takes the place of a regular vent through the roof. It's mounted just downstream from the trap. Do you need more information? Regards, tom