Security tags are labels which can be associated with a virtual machine or a group of virtual machines. Security tags are designed to be used with security groups. Once you create the security tags, you associate them with a security group which can be used in firewall rules. You can create, edit, or assign a user-defined security tag. You can also view which virtual machines or security groups have a particular security tag applied.
A common use case for security tags is to dynamically group objects to simplify firewall rules. For example, you might create several different security tags based on the type of activity you expect to occur on a given virtual machine. You create a security tag for database servers and another one for email servers. Then you apply the appropriate tag to virtual machines that house database servers or email servers. Later, you can assign the tag to a security group, and write a firewall rule against it, applying different security settings depending on whether the virtual machine is running a database server or an email server. Later, if you change the functionality of the virtual machine, you can remove the virtual machine from the security tag rather than editing the firewall rule.