Inventory Items on Service Orders (SiteLocationID)

What is the FSSODet.SiteLocationID default logic? Can this only default from the Inventory Item or can there be a customer or warehouse default?

When I add an Inventory Item to a Service Order the warehouse populated from Branch Location Default (i think) FSBranchLocation.DftSiteID but the SiteLocationID never defaults.

This isnt an issue for manual entry of Service Orders, I can just set the value. Adding stock items to Service Contracts...im running into issues generating the schedule.

Any guidance would be appreciated!