It compress any file into a seekable .zstd file
It compress a file into a seekable zstd splitting the file into multiple frames.
If the file is a tar archive it compress each file in the archive into an independent frame, hence the name: tar 2 seekable zstd.
This allows fast seeking and extraction of a single file without decompressing the whole archive.
The compressed archive can be uncompressed with any Zstandard tool, including zstd, and it can also be mounted with ratarmount.