A Threshold 
displays all of the control volumes where the value of a specified solution variable is inside (or outside) a chosen range.
You can use this tool to locate areas of interest within a flow domain.
Unlike an isosurface, which plots an interpolated surface, a threshold includes all of the control volumes within its range.
Click the threshold icon
in the visualization toolbar. In the properties panel, select the desired flow quantity from the Threshold Field dropdown.
Specify the Value Range.
The minimum and maximum values of the flow variable are shown to help you select appropriate numbers.(Optional) Select Smooth to create an interpolated, smoothed external surface for the threshold that doesn't necessarily follow control volume boundaries.
(Optional) Inclusive is selected by default. This option will display the control volumes with values inside the threshold. Deselecting this will display the control volumes outside.
(Optional) Select Strict to show only the control volumes for which the node values meet the threshold requirement.
Click the check button to apply the settings.
Navigate to the Visualizations section of the control panel and select the Threshold that you just created.
Use the dropdown menus in the Display section to adjust the final appearance of the threshold volume.
Note: Strict is not compatible with Smooth.