In App Studio, the Report canvas uses smart context functionality to determine what column type a field should be, given the metadata of the field and/or the position of the field on the canvas.
Depending on the metadata of the field, and whether you have the smart context option turned on or off, fields that you add to your report will be added as either an aggregate, BY, SUM, or Detail.
If smart context is on, and the metadata indicates that a field should be a measure or dimension, it will automatically be added as an aggregate or sort down field type, respectively.
If smart context is on, but the metadata does not indicate whether a field is a measure or dimension, then alphanumeric fields and date fields are added as BY fields, while numeric fields are added as SUM fields.
If smart context is off, all fields are added as DETAIL fields.
The smart context options are contained in the App Studio Options dialog box, in the General tab, as shown in the following image.