In SharePoint, when you run the Post Setup configuration tool, you might have noticed that it works differently for Single server scenario vs. Farm scenario. How does the PSConfigUI decide the steps that needs to be taken?

The answer is pretty simple, Registry Keys. The PSConfigUI reads the following keys in order to determine the correct mode in which it should run.

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Shared Tools\Web Server Extensions\12.0\WSS

SETUPTYPE = {CLEAN_INSTALL | REPAIR | V2V_INPLACE_UPGRADE | V2V_GRADUAL_UPGRADE | B2B_UPGRADE | UNINSTALL | SKU2SKU_UPGRADE}

SERVERROLE = {SINGLESERVER | WFE | APPLICATION}

Thought you should know!
Rahul