In IIS I can configure my custom error pages.
For each HTTP Error code I can say where to go. Several codes have a number of "sub" codes available. For example 404 has a regular 404, 404;1, 404;2 and so on..
What are they for? When are they returned? Should I make custom pages for these errors? Can I somehow easely configure all code "families" to come to the same page?
401 - Access denied. IIS defines several different 401 errors that indicate a more specific cause of the error. These specific error codes are displayed in the browser but are not displayed in the IIS log:
Here is the complete list in the MSDN documentation for IIS 5.0 and 6.0 and for IIS 7.0, 7.5 and 8.0.
If you want to show your visitors or users a nice custom message depending on these subcode, you could do it. But you needn't.