Numbering carbon atoms in biological molecules?

How do you number to carbon atoms in biological molecules like glucose? I know you get 1,4/1,6 glycosidic bonds for example, but what is it about the carbon atoms that makes them carbon 1,4 or 6?

It is convention to name carbon atoms starting at a particular atom (I googled "Standard Numbering System AND biochemistry" [boolean search] for you but there is no set of guidelines anywhere, nor in my volume on biochemistry by Voet et al [from which the diagrams below are taken]), and working in a certain direction in sequence - in the case of glucose, you start [with 1] at the carbon furthest from the -CH2OH [whose C atom is labelled 6] and work clockwise. (see pic below - new post from phone) - it seems like this system varies with the type of molecule (see diagram of cholesterol below):-

randombiochemist : are you able to help us with the criteria of the Standard Numberring System, please?