Halliday & Matthiessen (2014: 29):
If we start at the instance pole, we can study a single text, and then look for other texts that are like it according to certain criteria. When we study this sample of texts, we can identify patterns that they all share, and describe these in terms of a text type. By identifying a text type, we are moving along the cline of instantiation away from the text pole towards the system pole.