Indicates that Unity should not recalculate the bounds when you set Mesh data using Mesh.SetSubMesh.
When you use Mesh.SetSubMesh to modify a Mesh's data, Unity's default behaviour is to re-calculate the values of the SubMeshDescriptor.bounds, SubMeshDescriptor.startVertex and
SubMeshDescriptor.vertexCount fields from the current vertex and index buffer data.
You can make Unity skip these calculations by using the MeshUpdateFlags.DontRecalculateBounds flag. This can be beneficial to performance, however if you use this flag you must make sure that you pass correct values of the bounds, startVertex and vertexCount fields.
Additional resources: Mesh.SetSubMesh.