About Groups and Calculated Items
You can create a group or calculated item as a way to display data in a table, pivot table, trellis, heat matrix, or graph.
Groups and calculated items enable you to add new "members" to a column, when those members don’t exist in the data source. These members are also known as "custom members."
Use a group to define members of a column, as a list of members or a set of selection steps that generate a list of members. A group is represented as a member.
A calculated item is a computation between members, which is represented as a single member that can’t be drilled. When you create a calculated item, you add a new member in which you have selected how to aggregate the item. You can aggregate using Sum or Average or custom formula.