You can set tags to assign to a user/device (e.g. if user has done an action set a tag), so you can target those users later:

To set a tag or a group of tags:

MBAudience.addTag(context:Context, key: String, value: String)
MBAudience.addTags(context:Context, tags: ArrayList<MBTag>)

To remove it or clear all:

MBAudience.removeTag(context:Context, key: String)