Tableau divides the data in two main types: dimensions and measures. Dimensions are usually those fields that cannot be aggregated; measures, as its name suggests, are those fields that can be measured, aggregated, or used for mathematical operations. Dimension fields are usually used for row or column headings; measures are usually used for plotting or giving values to the sizes of markers.

When you import the data for the first time, Tableau determines whether to consider a field as a dimension or a measure. This determination involves considering fields with all text (nominal or other text) values and fields with numeric values. Depending on the data source, Tableau also uses Microsoft's Jet Engine Driver to classify fields into dimensions and measures.

Tableau visualizations are heavily dependent on the structure of dimensions and measures. Thus, organizing data properly into dimensions and measures is important, and if Tableau's determinations are wrong about the field data types, it is easy to convert these fields to the other category. Simply dragging the field to the pane works just like right-clicking on the field and clicking on Convert to Dimension or Convert to Measure.