# Glossary > Definitions of key terms used in Asset Transformer Studio | Item | Description | | --------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | | **Entity** | An *entity* is the base object in the API. All objects manipulated are derived from Entity (a part, a material, a texture… is an entity).See [Scripting With the Python API](../working-with-asset-transformer-studio/scripting-with-the-python-api). | | [**Product structure (tree)**](./about-the-product-structure-tree.md) | The [*Product Structure*](./about-the-product-structure-tree.md) designates the scene tree, or scene hierarchy. | | **Tree level** | The *tree level* (used in functions **Merge Final Level** and **Merge By Tree Level**) designates the level at which an occurrence is positioned in the Product Structure. The `Root` node is Level 0.![Tree level](/api/media?file=/asset-transformer-studio/2026.5.0/media/STUDIO_AddLevel.gif) | | **Final level** | In Asset Transformer Studio, a *final level* (used in function **Merge Final Level**) designates an `Assembly` node whose children are exclusively *part occurrences*.![Final level](/api/media?file=/asset-transformer-studio/2026.5.0/media/NewItem358.png) | | **Node** | A *node* is a single element of the Product Structure, more commonly called an *occurrence*. | | **Occurrence** | An *occurrence* is the fundamental object of a scene's Product Structure, similar to the [GameObject concept in Unity](https://docs.unity3d.com/Manual/class-GameObject.html). Each node of the Tree is an occurrence.See [About the Product Structure (Tree)](./about-the-product-structure-tree.md). | | **Part occurrence** | An occurrence with a *part component* is a *part occurrence*, more commonly called *part*. Basically, a *part* is any object containing geometry that can be selected individually in a scene.See [Occurrence Types](./about-occurrences/occurrence-types.md). | | **Part** | A *part* commonly designates a *part occurrence*. It can also designate a *part component* in the Python API. | | **Assembly** | An occurrence with no *part component*, and having other occurrences as children, is called an `Assembly` occurrence, or *assembly*.See [Occurrence Types](./about-occurrences/occurrence-types.md). | | **Instance** | An *instance* is an occurrence that is duplicated multiple times across a scene. In Asset Transformer Studio, instantiation is managed through a concept called *prototype*.See [About Instances & Prototypes](./about-instances-and-prototypes.md). | | **Component** | An occurrence can receive *components* to modulate its behavior (just like in [Unity](https://docs.unity3d.com/Manual/UsingComponents.html)).See [Occurrence Components](./about-occurrences/occurrence-components.md). | | **Prototype** | A *prototype* is an occurrence that can be referenced by other occurrences, allowing to create instantiation behavior. Occurrences pointing to a prototype are instances.See [About Instances & Prototypes](./about-instances-and-prototypes.md). | | **Empty** | The term *empty* (used for example in the function **Delete Empty Occurrences**) is associated with an occurrence containing no geometry (no *part component* or a *part component* containing no geometry), no light (no **Light** component) and having no children. | | **Metadata** | Metadata is non-geometric data that can be carried by a Product Structure occurrence, which was added by engineering for multiple possible reasons: design information, manufacturing information, logistic information… | | **Geometry** | A *geometry* is a collection of shapes. It can either be a CAD geometry (BRep shapes) or a Mesh geometry (Tessellated shapes).See [About 3D Models Types](./about-3d-models-types.md). | | **BRep** | **BRep** is the kind of geometry retrieved when importing a native CAD file. It is also called *CAD geometry* or *precise geometry*.See definition on [Wikipedia](https://en.wikipedia.org/wiki/Boundary_representation). | | **BRep shape** | In Asset Transformer Studio, a *BRep shape* contains BRep geometrical information like surfaces (or volumes), curves and points (themselves defined by faces, edges and vertices). | | **Patch** | A *patch* is a CAD face or surface limited by spline curves. | | **Patch boundary** | A *patch boundary* (or patch border) is a non-discontinuous patch border or edge.![Patch boundary](/api/media?file=/asset-transformer-studio/2026.5.0/media/PiXYZ-Software_AboutPatches.jpg) | | **Mesh** | See definition on [Wikipedia](https://en.wikipedia.org/wiki/Polygon_mesh). | | **Connected mesh** | A connected mesh is a separate mesh element within a Part. Think about 2 cubes merged together in one Part: each separate cube is a connected mesh. | | **Tessellation** | *Tessellation* is the method used to represent 3D objects as a collection of triangles or other polygons. The more triangles used to represent a surface, the more realistic the rendering, but the more computation is required.See [About Tessellation](./about-tessellation.md). | | **Tessellated shape** | A *tessellated shape* can contain meshes, free edges (a collection of connected free edges define a polyline, or curve) and free vertices. | | **Bounding box (BBox)** | See definition on [Wikipedia](https://en.wikipedia.org/wiki/Bounding_volume). | | **Axis-aligned bounding box (AABB)** | See definition on [Wikipedia](https://en.wikipedia.org/wiki/Axis-aligned_object). | | **Oriented bounding box (OBB)** | See definition on [Wikipedia](https://en.wikipedia.org/wiki/Bounding_volume). | | **Minimum bounding box (MBB)** | See definition on [Wikipedia](https://en.wikipedia.org/wiki/Minimum_bounding_box). |