This function (or case) indicates the mean by which something is done (by bus, by car, by bike, etc...) Here are some examples in Mongolian language.
a) Тэр автобусаар тийшээ явсан. He / She went there by bus.
b) Бид онгоцоор ирсэн. We came by plane.
c) Бид хөдөөгөөр aялмаар байна. We want to travel to the countryside (in the country).
d) Бид модоор байшин барна. We will build the building in wood.
e) Бид уржигдар мориор явсан. We went by horse the day before yesterday.
f) Бид монголоор хамт ярьцгаадаг. We speak in Mongolian together.
g) Та нар францаар түүнтэй ярьдаг уу? Do you speak french with him/her?