[A register is constituted by] the linguistic features which are typically associated with a configuration of situational features - with particular … - Michael Halliday

A child learning his [her] mother tongue is learning how to name; he [she] is building up a meaning potential in respect of a limited number of social functions. These functions [instrumental, regulatory, interactional, personal, heuristic, and imaginative] constitute the semiotic environment of a very small child, and may be thought of as universals of human culture

Language... is a range of possibilities, and an open end set of options in behavior that are available to the individual in his existence as social man. The context of culture is the environment of any particular selection that is made within them... The context of culture defines the potential, the range of possibilities that are open. The actual choice among these possibilities takes place within a given context of situation.

What makes learning possible is that the coding imposed by the mother tongue corresponds to a possible mode of perception and interpretation of the environment. A green car can be analysed experientially as carness qualified by greenness, if that is the way the system works.