Sur is the offering of scents. In the Buddhist worldview, there are many kinds of sentient beings. Some of them survive by eating solid food, some by consuming smell and some merely through touch or psychological nutrition through the force of the mind.
Smell is one of the many kinds of sustenance. It is particularly consumed by a category of sentient beings called dri za or smell consumers. An example of a dri za is the spirit of a dead person, which roams in the intermediate state of bardo between the previous and next lives. Bhutanese offer sur after someone has died because they believe that the spirit in bardo survives on smell.
There are two kinds of sur one can offer: vegetarian sur called kar sur and non-vegetarian sur called mar sur. In kar sur, one puts vegetarian substances including karsum or three white ingredients such as butter, milk and curd. Kapchye or roasted wheat or barley flour is used as the main base. Ngarsum or three sweet things, which are sugar, molass and honey are also added. Roasted barley flour is mixed with these ingredients to make sur to make the vegetarian sur. If one adds meat, blood or fat, that becomes non-vegetarian sur.
To some deities and spirits, one must offer the vegetarian sur and to some one can offer the non-vegetarian sur depending on the occasion. One can also add other substances to enrich the sur. This is the preparation of the raw materials during the offering of sur.