There are some interactive tools that are shared between Production Assistant and Ultimate S. The Montage, Motion, and Lower Thirds tool are in both as is the Cine Looks and Batch Rendering. Ultimate S, however, has 10 other tabs that are not in Production Assistant.
The difference with Production Assistant is that it is aimed at editors that could benefit from process automation. While in Ultimate S you can interactively perform an action and then another, in Production Assistant to can chain actions together and save those chains and then use them later on multiple files with no interaction from the editor. It's all automated.
BTW, Questions about Production Assistant do not need the "OT" in the subject as they are not "off topic". Production Assistant is a Sony plug-in to Vegas Pro and this is the correct forum to be asking questions about it.