Those are groups. Circular icon means cyclic group, the straight arrow means drill-down group. These groups are just groups of fields (or expressions that return a list of values as a calculated dimension) to make charts more powerful. You can see all of them and modify them in the Settings menu, Document Properties, Groups tab.
Hope that helps.
these dimensions are clustered in groups.
A circular is a group where you can change the dimension (eg from customer to artikel) with no selections.
A straight pointing down is a drill down group with selection the next level when selected one value (eg year -> quartal-> month -> week) with selecting the value.