The point of a USING statement is to inform the assembler which register to choose and which displacement to create when generating object code addresses. The IBM System/360 started with a single USING statement. Today, we have access to several different formats for creating USING statements. Are you familiar with them? If not, start here with a discussion of Labeled (or Named) USING statements. This kind of USING has distinct advantages over simple USING statements, especially in situations where the same data structure occurs multiple times and needs to be referenced simultaneously (as in linked lists). Here is some code you can use to play with these ideas.
Brush Up Your USING Skills
Filed under IBM Mainframe Assembler