The vendor details tab information is more relevant for automated replenishment and when the PO is generated automatically at various processing points. Here is relevant documentation link.

One may have to explore customization or changing automation step ( if permits ) to modify the behavior.

https://help-2020r1.acumatica.com/(W(1))/Help?ScreenId=ShowWiki&pageid=dfb8f3f0-81bd-43f3-a875-fdb5fa56d65d

thanks

everything looks like it should select the purchase unit specified in here on a PO raised to the specified supplier for the specified warehouse which is exactly what our client needs, but it doesn't. it picks up the purchase unit from the general settings tab and ignores the entry in here. 

has anyone had this working on a previous version or seen this working? the column is * like it's a critical field, the vendor is specified multiple times by default with just the unit being different (guessing) to give the different prices....

but when you raise the PO, you have to manually change the unit even if the only "active" unit is the required one. 

any advice would be appreciated. 

Thanks