Purpose shots is a general category of shots that are not any particular shot size, but are rather categorised by why they are there. Each shot in this section is used for a particular purpose to aid in telling a story. They are often effective by manipulating or aiding the audiences understanding of events that are portrayed.
This category is also closely connected to editing, as the editor decides what shots to put together In an order that best tells the story - but these techniques need to be in the directors tool-kit to ensure they are thinking of them when they are filming.