I have had this too and couldn’t think what it meant. I asked at the front desk last time I was going into the First lounge at B in DXB. The lady who was checking me in didn’t know but asked a supervisor. The supervisor asked if I had my Skywards account linked with my Marriott Bonvoy account, which I do, and she said she was pretty sure that’s what it was.

I believe there are some reciprocal benefits for Silver and above in both programs, but I don’t remember what they are.