The test plans are written in YAML format. They are human-readable and can also be utilized directly as release elements by the PLT Operating System. You may modify these in your own coding environment. If you use a Mac, you can also use Sublime Text editor or Atom to modify these files and re-save them as a test plan with a new name and/or version number. Atom also has distributions for Windows and Linux. Some people also use vscode which is a multi-platform code editor built by Microsoft.
We have a guide called the Test Plan Reference for the grammar of the YAML file as well.
Here are some online linters for validating the YAML file syntax:
Here is the official YAML website: https://yaml.org/
Here is a $6 book about writing in YAML: https://www.amazon.com/YAML-including-processing-generation-Python/dp/1703404602/ref=pd_lpo_14_t_0/144-8013520-1974312