Depending on the settings in message customizing it is possible to create a source list even without having any info record in your system.
Further, you allow variable order unit in your material master, along with possible alternative units defined there too.
Now you have several vendors for this material. You can now enter a source list with a different order units than specified in general in your material master, individually per source of supply.
And this will be taken into account when you create the PO.