This indicates an attempt to use Apple Messages.
Apple Messages is an instant messenger service developed by Apple Inc. for iOS and OS X. Messages allows users to send texts, documents, photos, videos, contact information, and group messages over Wi-Fi or Internet to other iOS and OS X users, providing an alternative to standard SMS/MMS messaging. All Messages are encrypted and can be tracked using delivery receipts. If the recipient enables Read Receipts, the sender will be able to see that the recipient has read the message.