### 10.4.2. Defining and Editing Composite Regions

The Regions details view is used to create new, and edit existing composite regions. To create a new region, select Composite Region from either the Insert > Regions menu or the shortcut menu available from Mesh in the Outline or Mesh trees. To edit an existing composite region, right-click it in the Outline or Mesh tree view, then select Edit Definition from the shortcut menu.

A composite region is defined by specifying a list of regions and a method for combining them. The Regions details view can be used to create or modify a region by selecting a method of combination from the Combination list and a selection of regions from the Region List.

Note: The Regions details view enables you to restrict the regions available for selection by limiting them by Dimension(Filter). Selecting `2D` will cause the Region List to only display 2D regions and selecting `3D` will cause the Region List to only select 3D regions. Regions of mixed dimensionality are always available.

#### 10.4.2.1. Union

A Combination setting of `Union` combines the area or volume of the selected regions to create a new region. The new region will include all the regions from which it is constructed. For example, two or more 3D regions can be combined to create a new region, which can then be used as the location for a domain.

#### 10.4.2.2. Alias

A Combination setting of `Alias` is used to produce a composite region that when resolved is based upon the same set of primitive regions as the region it is defined on. A composite region with a Combination of `Alias` may only reference a single region (this may be a composite or primitive region). The new composite region may, however, resolve to more than one primitive region. This feature is useful to assign recognizable names to regions with non-intuitive names.

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