Hello Michael
An excellent explanation of these notations can be found at the following link:
Note here that the (S, G) notation is used to refer to the shortest path tree (SPT) distribution tree. This is when no single shared root of the tree is being used. In other words, this is when no Rendevous Point (RP) is being used.
Whenever an RP is being used, then the notation changes to (,G), which refers to the fact that a Shared Distribution Tree (SDT) is being used. The (,G) notation always refers to this.
I hope this has been helpful!
Laz