### 9.8.1. Assemblies, Primitive Regions, and Composite Regions

Each mesh is imported into one or more assemblies. An assembly represents a connected mesh. A mesh containing one-to-one node connections is considered to be connected and is imported into a single assembly.

Each assembly contains one or more 3D primitives (mesh regions), and each 3D primitive is bounded by one or more 2D primitive mesh regions. Each 3D primitive may also contain 2D mesh primitives that are located within the interior of the mesh. A primitive is the lowest level of region information available in a mesh file.

Primitives could be regions that were explicitly created in the mesh generation software. However, in some mesh files, the mesh references underlying CAD faces, in which case these will be the primitive regions. GTM files are an example of this; a 2D primitive region will resolve to the CAD face `Solid 1.2`, for example. If CAD face data is available in the mesh file, then regions explicitly created in the mesh generation software, or in CFX-Pre, will reference the CAD faces and, therefore, themselves will not be the lowest level of region data. These regions are known as composite regions because they are composed of one or more primitive regions.

Note: Because CFX-Pre can recognize underlying CAD surfaces from CFX GTM Files, it is not necessary to create composite regions, although it will often make selecting locations easier in CFX-Pre. Other mesh types may or may not require the definition of composite regions within CFX-Pre.

New composite regions can be created in CFX-Pre using the Regions details view. However, the topology of the existing primitives limits the scope of composite region creation and it is not possible to create any new primitives in CFX-Pre. For details, see Defining and Editing Composite Regions.

The number and location of 2D primitives and 3D primitives is defined by the software that generated the mesh. You should consider your domain, boundary condition, domain interface and subdomain requirements when creating the mesh and create appropriate regions that can be used in CFX-Pre. You will need to create each region explicitly in the mesh generation software if your mesh file does not contain data that references the underlying CAD faces.

If primitives reference the underlying CAD faces, it does not mean that the exact CAD geometry is recovered. The mesh simply references all the CAD faces and makes the mesh associated with them available in CFX-Pre.

In CFX-Pre 3D primitives are always distinct, as such a mesh element is always contained in a single 3D primitive. All regions in the mesh file that define a set of 3D elements are imported into CFX-Pre. If any element exists in more than one grouping of elements, the import process will split the groupings so that each element is contained within a single 3D primitive. Composite regions will be defined that group the 3D primitives into the topology that the original mesh file represented. Depending on your mesh file, this could include 3D subregions, solid regions, block-off regions, user defined 3D regions, porous regions, and so on.

If a 2D primitive spans more than one 3D primitive, it will be split into multiple 2D primitives on import, so that each 2D primitive is part of only one 3D primitive. All overlapping 2D primitives are also split into distinct primitives upon import and composite regions are created to represent the original regions read from the mesh file. When a 2D primitive forms a boundary between 3D primitives, it will be split into two sides, such that a 2D primitive is associated with each 3D primitive. When a 2D primitive is split, a suffix is added to the name so that the resulting 2D primitives are named uniquely. For example, a 2D primitive called `Solid 1.2` would be split into `Solid 1.2A` and `Solid 1.2B`.

#### 9.8.1.1. Composite Regions

Composite regions are defined as combinations of one or more 2D primitive, 3D primitive or other composite regions. New composite regions created in CFX-Pre must therefore be defined by a combination of at least one other region, however it is possible that a composite region can be defined that resolves to nothing.

Composite regions that are specified in the original mesh file imported into CFX-Pre will be imported into the application if the import format can be translated into one that CFX-Pre can use. The composite regions imported into CFX-Pre can be selected, modified and deleted in the same way as composite regions defined in the application.

Additional information on primitive and composite regions is available in Assemblies, Primitive Regions, and Composite Regions.

For details about creating regions, see Regions.

##### 9.8.1.1.1. Applications of the Composite Regions

For details, see Applications of Composite Regions.

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