When Tidal let's you download a song/album for offline use, there must be a catch, right?
Is it still FLAC? Can I only use the download for a limited time? Could I download it to my house server and then use it like any other FLAC file?
I don't know if the format is FLAC, but it is encrypted and must be decrypted by using the Tidal software registered to an active account. Like Spotify, you are able to use offline mode for up to three devices per account.
The offline downloads are only available while an account is active for authentication.
There have been some third-party (illegal) downloading software like TiDown which converted the files into non-DRM MP3, but the music services usually move fairly fast to plug those security holes.