Following up, I found some information, on the bottom of page 2
File → Import Library
It is important to note that the DesignScript language is independent of Dynamo. For example, Dynamo also implements a number of useful
geometric and domain specific functions which can be programmed using DesignScript. We will use examples of these geometric classes and
methods to illustrate the features of the DesignScript language. However, it should be noted that these classes and methods are not part of
the DesignScript language. Other libraries, including geometry libraries can be built using C# development tools operating within the ‘Zero
Touch’ conventions, and the resulting DLL’s can be loaded into Dynamo using the ‘file/import’ command and accessed via the Dynamo UI and
DesignScript.
There are some aspects of DesignScript which are currently not supported through the Dynamo User Interface, but these can be accessed by
using a regular text editor to create a DesignScript source file [with the .ds extension]. These files can also be loaded using the Dynamo
‘file/import’ command.