The Auto Subtitles option is for audio/video files and generated text blocks to subtitles. This means that Auto Subtitles is available for all projects as long as you have at least one generated text block in Advance Mode.
TTS to Subtitles
The TTS to Subtitles option is only for text block to subtitles. If you have used Genny's voice generator to create your voiceover for your project in Advance Mode and have not uploaded any videos, then you will need to use this option. The subtitles will be generated based on the text blocks in your project.