Every data model has a given set of UI elements to address. Additionally you have the option to write your own custom templates for input controls or custom dashlets.
There are two files responsible for custom elements:
Put your html templates here. You can define templates for custom input controls like typeahead elements retrieving data from an API or codelist. You can also define custom dashlets here to use in the dashboard like attribute based charts or showing your current open tasks.
Put the related functions in this file. You can define your own REST calls within the BYRD API or third party APIs.
You can also overwrite system directives and templates to extend there functionality, like showing checkmark icons instead of checkboxes for boolean type attributes, etc.
But be aware that you can also break system functionality with this feature! So if something doesn't work as expected, check your data model and custom files first!