The true polymorph spell states:
Choose one creature or nonmagical object that you can see within range. You transform the creature into a different creature, the creature into an object, or the object into a creature (the object must be neither worn nor carried by another creature).
If you turn a creature into another kind of creature, the new form can be any kind you choose whose challenge rating is equal to or less than the target’s (or its level, if the target doesn’t have a challenge rating). The target’s game statistics, including mental ability scores, are replaced by the statistics of the new form. It retains its alignment and personality.
How should this be treated in the context of the rules, and how should it be treated differently (if at all) from other kinds of transformation, such as the polymorph and shapechange spells, and the Druid’s Wild Shape feature, all of which only use the phrasings “a beast”; “any beast”; “any creature”? Is there any creature (or beast) which this spell would therefore be unable to produce which the other transformations would, or is it redundant?
What is a kind of creature, and how do I determine what options a player has when he tries to use this spell?