This feature allows you to set alternatives [1].
For example, like this:

   @$(call add,ALTERNATIVES,/usr/bin/xvt:/usr/bin/xterm)

Also, in the config itself, targets for setting alternatives may already
be predefined. For example, use/alternatives/xvt/% allows you to specify
arbitrary alternatives for xvt:
use/alternatives/xvt/xterm, use/alternatives/xvt/mate-terminal, etc.
However, an alternative must be available.

1. https://www.altlinux.org/Alternatives