Correct: Separate taxonomic keys (like keys for family, genus, or species) are required for each taxonomic category.

Incorrect: Keys are based on a pair of contrasting (not similar) characters, known as a couplet.

Incorrect: Each statement of a key is called a lead, not a grade.

Incorrect: While keys are analytical in nature, there is no specific information in the search results that highlights the indented key specifically as the "most popular" type compared to the bracketed key. They are both common types of dichotomous keys.

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