TwlNmenu is the name for an application that comes bundled with TWLSDK's and the SystemUpdater. This application would install .TAD files, an installable archive developed by Nintendo, similar to the Nintendo Wii's .WAD archive.
.TAD File Format
The installable format, .TAD share many similarities to the .WAD format, including the same offsets for Title ID's and using the same devsign as WADS. A .TAD can be resigned using Wii resigning tools and a RVL devkey.
The .TAD format can be created using a tool included with the TWLSDK, named Maketad. Only applications that can be installed in the folder 00030004 can be created with this tool.