TagString: Value of the cat= paramter in the floodlight tag, which the ad servers use to identify the activity. This is optional: if empty, a new tag string will be generated for you. This string must be 1 to 8 characters long, with valid characters being [a-z][A-Z][0-9][-][ _ ]. This tag string must also be unique among activities of the same activity group. This field is read-only after insertion.