From Rare Gaming Dump

TwlNmenu is the name of an application that comes bundled with TWLSDKs and the SystemUpdater. This application installs TAD files, an installable archive developed by Nintendo, similar to the Nintendo Wii's WAD archives. TwlNmenu is able to install all types of NAND programs, however the NAND tab will only display user apps (TIDs starting with 00030004). This may have been done to prevent developers from erasing necessary system programs.

TwlNmenu's icon.

TAD File Format

The installable format, TAD, shares many similarities with the WAD format, including using the same offsets for title ID's and even using the same devsign as WADs. This can be explained by a commit to TwlIPL[1] where there's a mention of the WAD format being renamed to TAD.  Because TADs are derived from WADs, they can be resigned using Wii resigning tools and an RVL devkey; however, TwlNmenu has anti-tampering methods which will prevent you from installing resigned TAD files.

The TAD format can be created using a tool included with the TWLSDK, named Maketad[2].