Domain of a dependent USING instruction

The domain of a dependent USING instruction (called the dependent USING domain) begins where the dependent USING appears in the source module and continues until the end of the source module, except when:

You end the domain of the corresponding ordinary USING by specifying the base register or registers from the ordinary USING instruction in a subsequent DROP instruction.
You end the domain of the corresponding ordinary USING by specifying the same base register or registers from the ordinary USING instruction in a subsequent ordinary USING instruction.
You end the domain of a labeled dependent USING by specifying the label of the labeled dependent USING in the operand of a subsequent DROP instruction.
You end the domain of a labeled dependent USING by specifying the label of the labeled dependent USING in the operand of a subsequent labeled USING instruction.

When a labeled dependent USING domain is dropped, none of any subordinate USING domains are dropped. In the following example the labeled dependent USING BLBL1 is not dropped, even though it appears to be dependent on the USING ALBL2 that is being dropped:

ALBL1    USING           DSECTA,14
         USING           DSECTB,ALBL1.A
         .
         .
ALBL2    USING           DSECTA,ALBL1.A
         .
BLBL1    USING           DSECTA,ALBL2.A
         .
         DROP            ALBL2
         .
DSECTA   DSECT
A        DS              A
DSECTB   DSECT
B        DS              A

A dependent USING is not dependent on another dependent USING. It is dependent on the ordinary or labeled USING that is finally used to resolve the address. For example, the USING at BLBL1 is dependent on the ALBL1 USING.

Remember that all dependent USINGs must eventually be based on an ordinary or labeled USING that provides the base register used for base-displacement resolutions.