Transient meshes are meshes that are present during certain modeling phases but not others. Retained meshes, once created, are present for all steps of the modeling process unless you define a modeling entity that invalidates the mesh.

Transient meshes are convenient in situations where you want to experiment freely with meshing or where you plan to define all modeling entities and analyses before creating a mesh. However, if you want to re-use existing meshes or expect to work with modeling entities and analyses after you mesh, you should consider setting your session to use retained meshes.

By default, Creo Simulate uses transient meshes for FEM mode. If you want to use retained meshing, you need to set the fem_hierarchical_mesh configuration option in your configuration option file before you start your session. Be aware that this also affects whether Creo Simulate automatically saves and retrieves your mesh.

Your decision on whether to set your session for transient or retained meshing affects the simulation modeling process, determines the assembly meshing methods you can use, and involves distinct usage guidelines. You may find that you want to work with transient meshes for certain model types or during certain modeling phases, but switch to retained meshes for other model types or modeling phases. Switching between the two forms of meshing can make modeling more efficient in some situations, particularly if you determine how you want to handle your model meshes in advance.

To learn more about these mesh types, their workflow, and the guidelines you should observe, see If You Use Transient Meshes and If You Use Retained Meshes.