As documented in the Document Style Setup section, the filters can be setup in the "Doc. Type Assignment" to automatically find the most relevant Document Style when a new document is created. In addition to this (introduced in Document Delivery V3.0.4) the system will also use the same "Doc. Type Assignment" setup against Posted Document Types (e.g. Posted Sales Invoices) to enable the Document Style to be changed on the Posted Document. 

A scenario where this could be used is where you multiple Brands within a single Company and wish to use a different Document Styles for the Sales Order based on a Dimension Code that represents each Brand, but when you post the Invoice for the Sales Order you want to send different Invoice Document Styles based on whether the Customer is a Domestic or Export Customer. 

To achieve this simply setup the "Doc. Type Assignment" as required against both the unposted and posted Document Delivery "Document Types". 

It is not mandatory to setup "Doc. Type Assignment" filters against posted "Document Types" and if do not then the system will continue to use the Style Code to find the relevant "Document Type" at the point of posting, as detailed in the Using Document Styles section.