Chain
Chain is a simplistic scripting language. It's how you add custom logic and behaviors to maps, by utilizing the features of the language itself alongside Actions and Routines. Chain can be used as a standalone feature in game development, but ChainLink is a runtime for Chain that provides helpful abstractions which map developers can make use of.
You can learn more about how to use and work with Chain in its documentation HERE. This includes Calls and Targets!
Usage in ChainLink
Chain is primarily used to execute Actions and Routines in a certain order. In many places throughout the system of ChainLink, there is an available space to put a "Chain script" for execution, typically whenever something is activated.
Chain Calls can be Actions or Routines. For Chain Targets, you can use Link Ids.