Specify the name you wish to give this tag.
You can embed the following variables with groovy string format.

env

Map returned by hudson.model.AbstractBuild.getEnvVar().

sys

Properties returned by System.getProperties().

For example, you can embed JOB_NAME in the tag name, and add groovy expressions as well.

${env['JOB_NAME']}-${new java.text.SimpleDateFormat("yyyy-MM-dd").format(new Date())}

This gets the current date and includes it in the tag name.