The is example demonstrates very clearly the IMAQ/Vision ROI Descriptor. In the VI you learn about each part of the ROI descriptor and how each shape in an ROI is defined.
This example demonstrates how each ROI type is defined. There is a tab for each ROI type and the last tab allows you to draw an ROI and see how the ROI descriptor is changed. Please use the last tab to understand the difference between Freehand ROI, Roken Line ROI, Freehand Line ROI, and Polygon ROI. As these 4 ROI types differ more in how they are drawn on the IMAQ Image control and less in how they are defined in the ROI Descriptor cluster.
Example code from the Example Code Exchange in the NI Community is licensed with the MIT license.