Top View Node
When you create a new calculation view there are always two nodes that are automatically provided.
The very top node is the Semantic node. This node is used to define enrichments to the final results set, such as adding currency information to measures, or apply masking rules to cover up sensitive parts of a data. This is a mandatory node and no additional nodes can be added on top.
Below the semantic node, there is another very important node called the top view node. This node generates the calculated data set before semantics are applied.
The type of top view node that is automatically provided depends on the type of calculation view you are creating. SAP provide a default top view node type for each of the calculation view types, but it is possible to swap this default node to an different type. You would do this to modify the behavior of your calculation view so that the correct final result is produced before the additional semantics are applied.
Used in conjunction with the switch of calculation view data category, which you find under Properties, it means you can avoid having to create a brand new calculation view if you just want to change the final step in data preparation. When you switch node type the graphical calculation view editor attempts to preserve as much of the existing calculation view definition as possible.
You cannot remove the top view node or add any additional nodes on top of it.
A very simple calculation view might not require any additional nodes below the top view node.