What is a Tableau data pane?

The Data pane is the panel on the left of a worksheet listing every field in the connected data source.

Fields are split into dimensions (blue, qualitative) and measures (green, quantitative), along with any parameters and sets you have created. Tableau also generates some fields itself — Measure Names, Measure Values, Number of Records, and Latitude and Longitude where a geographic role is assigned.

You build views by dragging fields from here onto shelves. Right-clicking a field gives the operations that belong to it: create a calculated field, group, set, hierarchy or bin; change data type or default aggregation; assign a geographic role; rename or hide.

Anything you change here applies wherever the data source is used, which is why default formatting and sensible field names are worth setting once rather than per sheet.