Purpose
This link explains the standard system behvior of "long Text" Button in Enjoy Transactions
Overview
You define some additional Text IDs for line items with transaction OBT10
You find the "Long Text" button is only available in Classic Transaction(i.e. FB01) but not available in Enjoy Transaction.(i.e. FB60)
This is SAP standard program behaviour since early ENJOY times, and this construction is due to the particular architecture of Enjoy Transaction