In case of an open variant or alternative reading, the inline variant is encoded as a <seg> with a @type attribute "alternative". The other variant is encoded as an addition with a @type attribute. Both the <seg type="alternative"> and the <add type="alternative"> need an xml:id.
For example:
[xml]That is <seg type="alternative" xml:id="alt1">an easy thing to say</seg>
<add place="supralinear" xml:id="alt2" type="alternative">soon said</add>.[/xml]