Domain of a labeled USING instruction

The domain of a labeled USING instruction (called the labeled USING domain) begins where the USING instruction appears in a source module. It continues to the end of the source module, except when:

A subsequent DROP instruction specifies the label used in the preceding labeled USING instruction.
A subsequent USING instruction specifies the same label used in the preceding labeled USING instruction. The second specification of the label causes the assembler to end the domain of the prior USING with the same label.

You can specify the same base register or registers in any number of labeled USING instructions. However, unlike ordinary USING instructions, as long as all the labeled USINGs have unique labels, the assembler considers the domains of all the labeled USINGs to be active and their labels eligible to be used as symbol qualifiers. With ordinary USINGs, when you specify the same base register in a subsequent USING instruction, the domain of the prior USING is ended.

The assembler converts implicit address references into their explicit form using the base register or registers specified in a labeled USING instruction when the following conditions are met:

The address reference appears in the domain of the labeled USING instruction.
The address reference takes the form of a qualified symbol and the qualifier is the label of the labeled USING instruction.
The address lies within the range of the labeled USING instruction.