Reference
Tables to look things up quickly: formats, limits, defaults and file layouts. Where a value depends on the game version, the page says which version it was checked against. 9 reference pages so far.
Players and UUIDs
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The four ways Java Edition writes a UUID, how account and offline-mode UUIDs are generated, and where each form appears in commands, files and NBT.
Skins
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Skin image sizes, where each body part and outer layer sits in the 64 × 64 texture, and how slim arms and old 64 × 32 skins differ.
Running a server
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Default ports and protocols for Java and Bedrock servers, RCON, query and the management protocol, plus how SRV records let players skip the port.
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Every setting in a vanilla Java Edition server.properties file, with its default value, allowed values and a short explanation.
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The JSON files a Java Edition server uses for its whitelist, operators, bans and name cache, with the fields each entry contains.
Text, colours and MOTDs
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Section-sign codes, ampersand codes, hex colour codes, MiniMessage and JSON text components compared: what each looks like and where it works.
Coordinates and chunks
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Which way X and Z point, the height range of each dimension, the world border and the Nether coordinate scale, in one table.
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Formulas for converting block coordinates to chunk, section and region coordinates, with worked examples for positive and negative values.
Items and storage
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Which kinds of items stack to 64, 16 or 1, and how many items fit in a shulker box, chest, double chest and inventory for each stack size.