Packing

Packing is the process of embedding custom content into a map file so that other players can see the content, without having to separately download any other files like textures or models. If packing is not done, other people will not be able to see the custom content unless they also happen to have it on their end.

Programs that can pack content include:
 * - Valve's official tool
 * - Packing is 100% manual. Other programs are automated to some degree.
 * - Comes with tools besides packing. Somewhat outdated but usually works.
 * - Mainly geared towards map compiling but can also pack at the same time. Actively maintained.
 * - Very old packer, known to corrupt the BSP under certain conditions.

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In Portal 2, a voiceline spoken by Dr. Kleiner from Half-Life 2 will play in place of missing sounds. CS:GO uses a buzzer sound for the same purpose. Sometimes when a texture (or possibly, shader) is missing, it will display in a wireframe fashion seen in this video thumbnail.