Hello Yuanaw
In Application dictionary, many tables or structures have two point of view, one of sales and the other is purchase view, in elements you can define different text for this point of view, in the first part are the sales texts and in second part the PO (purchase order) texts
one example to understand this, are the m_inout table, for the view of sales are shipments, for view of purchases are receipts, in c_invoice table the view of sales are customers invoices, for the view of purchase are vendor invoices.
this is used in translations too, in windows definition you can chose if the windows is for sales or purchase selecting the Sales Transaction Checkbox