what is this cursor?

When you dimension entities and a center line/construction line then you see this symbol.

R is for radius and D is for Diameter. Depending on your first dimension, the cursor would to R or D so that you don't have to select the center line/construction line again when dealing with other entities. It will stick to same behavior until you click ESC or select two entities.

Added video for better explanation. Check the D and R symbol next to cursor.