The next step is to define the active cells in the model. The cells inside the domain outline should be active and those outside should be inactive. By default, all the cells are active. The first step is to set all the cells to inactive. Select Data|Edit Data Sets... And then select the Active data set in the tree control on the left. It is under "Required|Hydrology." When the Active data set is selected, the field for the default formula will become enabled and show "True." Change the default formula to "False" and click the Apply button.
The next step is to make all the cells inside the domain outline active. Double-click on the domain outline object to display the Object Properties dialog box. Note that Number of Z formulas is set to Zero. This means that the domain outline object is two dimensional but it needs to be three-dimensional to set the values of the Active data set because the Active data set is a three-dimensional data set. Change Number of Z formulas to Two. The edit boxes for the Higher Z-coordinate and Lower Z-coordinate will become enabled. Check that the formulas for them are "Model_Top" and "Aquifer_Bottom" respectively. This means that the domain outline object now extends vertically from the top of the model to the bottom of the aquifer. Now switch to the Data Sets tab and check the check box next to the Active data set. It is under "Required|Hydrology." The edit box for the Active data set formula will become enabled. Change the formula from "False" to "True." and click the OK button to close the Object Properties dialog box.
Now that the domain outline object has been used to set the Active data set, it is no longer necessary to see it so hide it by right-clicking on the top view of the model and selecting Hide Selected Objects from the pop-up menu.
To check that the Active data set has been set correctly, select Data|Color Grid... or click the Data Visualization button 
to display the Color Grid pane of the Data Visualization dialog box. Change the Data set of boundary condition to the Active data set (under Data Sets|Required|Hydrology) and click the Apply button. The cells inside the domain outline should be active (blue) and those outside should be inactive (red).
