The domain of an ordinary USING instruction (called the "ordinary USING domain", or the "USING domain") begins where the USING instruction appears in a source module. It continues until the end of a source module, except when:

• A subsequent DROP instruction specifies the same base register or registers assigned by a preceding USING instruction.
• A subsequent USING instruction specifies the same register or registers assigned by a preceding USING instruction.

The assembler converts implicit address references into their explicit form when the following conditions are met:

• The address reference appears in the domain of a USING instruction.
• The addresses referred to lie within the range of some USING instruction.