When you create a part, it exists in its own coordinate system, independent of other parts in the model. In contrast, you use the Assembly module to create instances of your parts and to position the instances relative to each other in a global coordinate system, thus creating the assembly. You position part instances by sequentially applying position constraints that align selected faces, edges, or vertices or by applying simple translations and rotations.

An instance maintains its association with the original part. If the geometry of a part changes, ABAQUS/CAE automatically updates all instances of the part to reflect these changes. You cannot edit the geometry of a part instance directly.

A model can contain many parts, and a part can be instanced many times in the assembly; however, a model contains only one assembly. Loads, boundary conditions, predefined fields, and meshes are all applied to the assembly. Even if your model consists of only a single part, you must still create an assembly that consists of just a single instance of that part.

A part instance can be thought of as a representation of the original part. You can create either independent or dependent part instances. An independent instance is effectively a copy of the part. A dependent instance is only a pointer to the part, partition, or virtual topology; and as a result, you cannot mesh a dependent instance. However, you can mesh the original part from which the instance was derived, in which case ABAQUS/CAE applies the same mesh to each dependent instance of the part.