With render itterations etc…?
All modifiers and objects that have Viewport / Render settings are supported according to the saver’s settings - Proxy meshes are saved in viewport mode, Render meshes are saved in render mode. This also applies to the “Enabled in Renderer/Enabled in Viewport” icon of ALL modifiers, so even if a modifier does not support separate Render/Viewport settings, you can turn it on or off in either output.
Objects like Particle Flow and Frost that have separate Viewport/Render settings will also be respected.
So if you have a Turbosmooth set to 0 iterations in viewport and 2 iterations in Render, saving with Proxy will give you a low-res non-smoothed version in the Proxy sequence and a high-resolution mesh in the render sequence.
As mentioned many times, XMesh was designed to capture the exact geometry that goes to the renderer. When saving a Proxy sequence, we simply tell Max not to switch to Render mode, so all the meshes are produced with their viewport settings. This is controlled by the MAXScript methods
XMeshSaverUtils.SetSceneRenderBegin()
...
XMeshSaverUtils.SetSceneRenderEnd()
So if you are scripting your own XMesh Saver, you have to call these to turn the render mode on and off, otherwise the viewport settings will be used.