Twitter ads targeting for Flexible Audiences and Similar audiences

For some reason, we can't select tailored audiences when we browse them on the page to create flexible audiences, why so? How can we find them?

View attachment 244202

On the other hand, at the moment to create a campaign from a Tailored Audience just below it appears an option that says "Target similar people to those in your chosen audiences". What is the difference between ticking this option and selecting "Expand your audience features" that appears at the end of the Campaign...

Twitter ads targeting for Flexible Audiences and Similar audiences

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