Purchase Order vs Purchase Invoice vs Purchase Journal


I would like to know when to use purchase order, purchase invoice and purchase Journal since all of them have the same function.

In short, in what is the difference between these 3?

Thank you


Functionality and ease of use.

Generally it is historical remains from those times, wen Navision license was granule based. Small company could buy Journal only, as it was cheaper and you still can enter Purchases. PO/PI offers additional possibilities - as creating PO from Quote. copying new PO from different posted docs, etc, etc.

Theoretically, you can enter ALL transactions in a single GL Journal, no need of all the docs and other Journals, but in real life it is true only for a small company.