TBA stands for 'to be announced' is used as a placeholder term when the information is expected to become known later on. It would most commonly be used when the title of the next episode has not yet been made public through such methods as a promo, press release, social media, interview with cast, crew or special guest star, so TBA should be written in. This not only fills up the blank space, but helps prevent information that might be unintentionally inaccurate or deliberately false from getting added.
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