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(nasm.info)Section 6.2.2

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6.2.2. `GROUP': Defining Groups of Segments
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   The `obj' format also allows segments to be grouped, so that a single
segment register can be used to refer to all the segments in a group.
NASM therefore supplies the `GROUP' directive, whereby you can code

     segment data
     
             ; some data
     
     segment bss
     
             ; some uninitialised data
     
     group dgroup data bss

   which will define a group called `dgroup' to contain the segments
`data' and `bss'. Like `SEGMENT', `GROUP' causes the group name to be
defined as a symbol, so that you can refer to a variable `var' in the
`data' segment as `var wrt data' or as `var wrt dgroup', depending on
which segment value is currently in your segment register.

   If you just refer to `var', however, and `var' is declared in a
segment which is part of a group, then NASM will default to giving you
the offset of `var' from the beginning of the _group_, not the
_segment_. Therefore `SEG var', also, will return the group base rather
than the segment base.

   NASM will allow a segment to be part of more than one group, but will
generate a warning if you do this. Variables declared in a segment
which is part of more than one group will default to being relative to
the first group that was defined to contain the segment.

   A group does not have to contain any segments; you can still make
`WRT' references to a group which does not contain the variable you are
referring to. OS/2, for example, defines the special group `FLAT' with
no segments in it.