Regional transfers are a special mechanic specific to the relationship between a regional center and its territories in regions.
They allow to setup automatic transfers of production from the sub-territory to its regional center for free and as a constant flux transfer instead of through trading events.
Regional transfer only become available after Barter.
These transfers are one-way only: from sub-territory to center. To transfer non-building-materials the other way, the sub-territory will need wither 1. supplies by army; 2. own Market and purchase goods from the region center through normal trading (usually with internal orders) or 3. with portage.