IBM Assembler Question:

neither DROP nor USING affect the register contents.
Once a register has been loaded with the address of a piece
of storage, the using statement can be used to 'map' that
storage against a set of labels defining the layout of that
storage e.g. a DSECT. Then whenever one of those labels is
referenced in the code, in moves etc, the assembler resolves
the relative address as a particular displacement from the
absolute address in the register. The DROP instruction
removes the relationship between the labels and the register
and renders subsequent references to those labels as
unresolvable, giving rise to errors at assembly (compile)time.
Typically the DROP instruction will be used to allow use of
the register for another purpose, e.g. address a different
bit of storage via a using staement on second DSECT without
the risk of corrupting that data via moves referencing the
original DSECT.