Is this documented somewhere? I can see the obvious nature of this, marking is to transpose costs, and in a standard environment then the costs are always the same, but you also use marking to create a relationship, which is still useful!
Yes it is documented - I have seen this in a white paper - Cost Management.
But I think the primary and main objective of marking is to play around the costs.
If you want to have relation ship like for which so the po has been created - create a po directly from so - the system will have a relationship even though it is standard cost enabled item…