Only the top level ad units. Metrics include events for their
descendants that are not filtered out.
FLAT
All the ad units. Metrics do not include events for the descendants.
HIERARCHICAL
Use the ad unit hierarchy. There will be as many ad unit columns as
levels of ad units in the generated report:
The column Dimension.AD_UNIT_NAME is replaced with columns "Ad unit 1", "Ad
unit 2", ... "Ad unit n". If level is not applicable to a row,
"N/A" is returned as the value.
The column Dimension.AD_UNIT_ID is replaced with columns "Ad unit ID 1",
"Ad unit ID 2", ... "Ad unit ID n". If level is not applicable to a
row, "N/A" is returned as the value.
Metrics do not include events for the descendants.