The Hashicorp Configuration Language (HCL) is used to define extensions in human readable way.

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HCL Basics

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Attributes

In HCL, an attribute is a fundamental element that assigns a value to a name. Attributes are key-value pairs, and their values can be simple literals (like strings or numbers), or they can be more complex expressions. For example: 
field1 = "a field"
field2 = 2

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Blocks

blocks, in HCL, are structures representing a higher-order configuration container that can hold attributes and, in some cases, nested blocks. Blocks form the core structure of HCL-based configurations, especially in tools like Terraform, and in Scale Signatures and Extensions. A block in HCL has a type, zero or more labels, and a body. The type defines the kind of configuration the block will hold (like a resource, or in the case below, a model). Labels are additional identifiers to distinguish or parameterize the block further. The body of the block contains attributes and possibly nested blocks. 
model Thing {
  string "name" {
    default  = ""
    accessor = true
  }
}
the types of the blocks for extensions include: interface, function, enum, model.

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Example Extensions