Halliday & Matthiessen (2014: 28-9):
System and text are thus related through instantiation. Like the relationship between climate and weather, the relationship between system and text is a cline – the cline of instantiation. System and text define the two poles of the cline – that of the overall potential and that of a particular instance. Between these two poles there are intermediate patterns. These patterns can be viewed either from the system pole as sub-systems or from the instance pole as instance types.