Sometimes (not very often) the Lotus Domino server crashes. And if its your code that does that (and it is possible to write code that will crash a server) its very useful to get to the bottom of it as quickly as you can. Domino (and Notes) when it crashes produces an NSD file - a text file with a huge amount of information.
There's a really good handout from Lotusphere 2008 you can download from here which goes into a lot of detail, and you can download the updated Lotus Notes Diagnostic tool (v2.7) from here. The 2.7 release is dated 15th March 2009, so I'm guessing its newer than the one you might already be running.