Blue Checkmark next to SMS message on only one contact

[AC Question]

I started to notice with SMS a blue or grey checkmark next to one of my contacts when I text them. Sometimes it will show them even typing a reply with a pop up that says "typing".... But the check mark doesnt appear with all my contacts and even on the one it does show only sometimes will it show this feature. Does this mean this contact is on some other messging device when I am texting this person or?? I hope this makes sense.