Tag (item)

In survival, a stonecutter can be used to cut a name tag into either 16 "Name" or 16 "Tag" items. These can be cut into 16 Bit items each.

Bits can be used in a crafting table to craft Left Curly, Right Curly, Left Square, and Right Square. In appears like these are the only tag items that can be crafted from Bits.

A crafting table can be used to convert a "Tag" item into a "Byte Tag". Two Bytes can be craftet into a "Short Tag" (equivalent to 16 bit), two Shorts into an "Int Tag" (equivalent to 32 bit), and two Ints into a "Long Tag" (equivalent to 64 bit). Many other combinations that form either 16, 32, or 64 bit are possible. For example, two Shorts and an Int make a Long. Undoing any of these crafting actions in e.g. a stonecutter is not possible.

Float, Double, and String appear to be unobtainable in survival.

"Name" and "String" items must be named in an anvil before they can be used. Unnamed "Name" and "String" items always produce a "Sssyntax Error". None of the other tag items can be named. It appears like there is especially no way to give any of the numeric tag items a non-zero value, despite them obviously holding their value in an internal NBT tag (this can be seen when advanced tooltips are enabled).

All these items can be combined in a crafting table to form a short sequence of JSON code, as long as the resulting syntax is valid. For example:
 * A pair of Curlies that enclose a named "Name" item and some value, e.g. a Byte, create a "Compound Tag" with the content.
 * Because of the limited size of the crafting table the maximum number of name-value pairs that can be put between Curlies is 3.
 * An empty pair of Curlies produces an empty Compound Tag with the content.
 * A pair of Square brackets can enclose up to 7 values, e.g. 7 Bytes. This produces a "List Tag" with the content.
 * "Name" items cannot be used in a List.
 * An empty pair of Square brackets produces an empty List Tag with the content.

The resulting Compound and List Tags can be used as values in other Compound and List Tags, as deeply nested as you want.

Tags appear to have no further functionality. They cannot be eaten or placed in the world, except in an item frame, as each other item can.

Trivia

 * Even if not explicitly mentioned all this is obviously meant to create NBT tags with the game's crafting system.
 * This implies that a Name Tag is 128 bit.

Gallery
Tag (item)