Placemark supports a nearly-unlimited range of data types for properties. You can name properties anything you want, and store any type that can be encoded as JSON - text, numbers, objects, arrays, boolean values.
But in some cases, you want to easily input a predictable set of properties with a form. For that, Placemark has: forms. You can create a form by clicking the gear icon next to the properties editor.
Then you can create a form, and after saving the form, alongside Raw, you have "Form" option, which contains the simplified fields.
Forms have to interact with pre-existing data, because you can create and modify then at any time. Here's ho they do that: