Portal 2 VCDs
Valve uses Squirrel scripts to make Glados (and other characters) talk throughout Portal 2. This is done by the GladosPlayVCD() function. Each piece of dialogue is associated with a unique key. However, Hammer is unable to pass this key to the script (it would require wrapping in quotes, which would mess up the VMF structure). So, Valve also associated dialogue snippets with numbers (or indices) which Hammer can store just fine.
These pages serve as a reference to allow mappers to re-use voice lines as they see fit, by mapping indices to voice lines.