Marking Cells for Region Sources
Typically it is not necessary to apply a region source to an entire region, but to a small zone of cells within the region. It is possible to use cell sets, and their associated field functions, to mark such a region of cells.
In the example that is shown below, it is desired to set an energy source on a vertical column of cells. The cell set defining this can be created using a geometric range where the limits are set just outside the column of interest.

The cell set was created on the Volume Mesh representation.

This cell set, which has been named Cell Set 1 by default, has a corresponding field function whose Function Name is a contracted form of the cell set name. In this instance, the Function Name is CellSetVar_1.

To use the cell set in another field function expression, simply call $CellSetVar_1. This call returns the value of 1 for all cells that are contained within the cell set, and 0 otherwise.