What does the + sign mean on an Invisalign tray?

I recently noticed my last three trays have the + sign on them. I.e. 47+, 48+, 49+. What does the + mean? Additionally, is it realistic to assume that any dentist will remove these attachments for me when this is over? I moved mid-treatment and have not found a new dentist yet. My attachments all fit perfectly and my teeth look great so I'm not worried about it.