In order to get the min, max of a quantity on a face region, you can use the macro MinMaxValueFaceRegion.PFM. For instance one would like to get the max value of B of a isovalues display on a face region. For that, you have first to display the isovalues of B and then execute the macro.

In order to get the min, max of a quantity on a volume region, you can use the macro MinMaxValueVolumeRegion.PFM.

In transient application, there is also the command Spatial transient computation (min, max, average) in the Advanced menu.