This is just a Yellowfin paradigm. Flagging a column as a metric will allow you to aggregate that column at the report level, ie Sum, Average, Min, Max. It also effects some filtering options, for instance, prompt values will not be shown when filtering by a metric.
You may have a Column such as "PersonId". This could be an Integer, but it is not a metric. It doesn't make sense to Sum or Average this column, as it is a dimensional value that refers to a particular person entry. Another way of looking at this is that Dimensional data is fairly static, we measure things against dimensions. For instance we could measure "AccountBalance" against "PersonId".
- "AccountBalance" is a dynamic value.
- "PersonId" will always represent a particular person in the database.
Because of the static nature of dimensions, we only allow hierarchies to be created against these columns. You can't set up a hierarchy on metrics. You can choose to use a numeric field as a dimension, though.