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(nasm.info)Section 6.2.7

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6.2.7. `obj' Extensions to the `EXTERN' Directive
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   If you declare an external symbol with the directive

         extern  foo

   then references such as `mov ax,foo' will give you the offset of
`foo' from its preferred segment base (as specified in whichever module
`foo' is actually defined in). So to access the contents of `foo' you
will usually need to do something like

             mov     ax,seg foo      ; get preferred segment base
             mov     es,ax           ; move it into ES
             mov     ax,[es:foo]     ; and use offset `foo' from it

   This is a little unwieldy, particularly if you know that an external
is going to be accessible from a given segment or group, say `dgroup'.
So if `DS' already contained `dgroup', you could simply code

             mov     ax,[foo wrt dgroup]

   However, having to type this every time you want to access `foo' can
be a pain; so NASM allows you to declare `foo' in the alternative form

         extern  foo:wrt dgroup

   This form causes NASM to pretend that the preferred segment base of
`foo' is in fact `dgroup'; so the expression `seg foo' will now return
`dgroup', and the expression `foo' is equivalent to `foo wrt dgroup'.

   This default-`WRT' mechanism can be used to make externals appear to
be relative to any group or segment in your program. It can also be
applied to common variables: see *Note Section 6.2.8::.