/meta is a directory that is used by the NandSDWorker. NandSDWorker loads banners from the title.met files under here if the banner is missing from the title contents, specifically, if content 0 could not be opened or if it is smaller than 0x50000 bytes. System Menu 1.0 only reads banners from here, so all banners must be copied here for channels to work under System Menu 1.0.
This directory has a similar layout to /title; there are subdirectories for the upper bits of title IDs, and subdirectories within those for the individual titles. Each title's directory only contains title.met.
Some SDK versions seem to write WAD footers to this area of the NAND; this footer typically consists of an internal name of the software, possibly with a build tag as well, all padded to 0x20 bytes, and then the title ID and version, padded with zeros to a total file size of 0x40 bytes.
Nintendo may have intended to put other misc. contents in /meta, which is why they used an entire directory for every meta entry. However, everything instead ended up in the main title contents.
