Spotify generates lof of playlists based on your Spotify favorite songs, also lists of recommended songs. There are "Daily Mixes", "Discover Weekly" playlists, and many others.
To manage them with Soundiiz, you must "follow" these lists first on Spotify. Then you will see them on Soundiiz, ready to be exported, transferred, or used in a Sync!
You can also copy the playlist URL (use the "Share" button on Spotify) and use this link as a source URL in Sync or use "Import playlist" and "By URL" on Soundiiz interface.