Range of a dependent USING instruction

The range of a dependent USING instruction (called the dependent USING range) is either the range as specified by the range end, or the range of the corresponding USING minus the offset of address within that range, whichever is the lesser. If the corresponding labeled or ordinary USING assigns more than one base register, the maximum dependent USING range is the composite USING range of the labeled or ordinary USING.

If the dependent USING instruction specifies a supporting base address that is within the range of more than one ordinary USING, the assembler determines which base register to use during base-displacement resolution as follows:

The assembler computes displacements from the ordinary USING base address that gives the smallest displacement, and uses the corresponding base register.
If more than one ordinary USING gives the smallest displacement, the assembler uses the higher-numbered register for assembling addresses within the coinciding USING ranges.