Difference between door casing and lining

[spada01]

Will shortly be building a new porch with external door. Will go for 1981 by 838 door. I've always used linings previously inside but do I need a door casing for an external door? What's the difference?

Also, do I fit the frame only when the new blockwork has been rendered?

 

[foxhole]

Casing is one piece , no loose door stop to reduce security[and more weather proof].If you fit frame first you can tack on a temp timber to act as a stop for the render to give it a neat edge.