lucy24 - 1:03 am on Jan 10, 2013 (gmt 0)
DirectoryIndex finds the right internal index file when URL with trailing slash is requested.
Where "finds the right internal index file" is short for:
... searches the list of names given in the DirectoryIndex directive, checks them against the contents of the requested directory, and uses the first one it finds.
And if it doesn't find one, it proceeds to Option B: check whether auto-indexing (Indexes option) is enabled for the current directory, either explicitly or by inheritance.
Details like this can matter if you're on shared hosting, because the host may or may not divulge what the config-file settings are.