The custom property may be a document property, if it is defined with the Document Properties Marker Macro on a Confluence page or a space property if it is defined with the same macro, but the page is the homepage of a space.
A space blueprints is a tool to help users bootstrap the space creation process. From a dialog users select a space blueprint and create a new space with a basic, blueprint-specific set of pages.
Wiki Markup is a special syntax used in Confluence to describe pages. At specific locations this syntax is the only way to add macros. This is the only use case for this kind of markup in the context of the projectdoc Toolbox.
Properties are metadata that can be added to every projectdoc document. If you require a set of metadata for each instance of a document type, you should write your own doctype.
We like to think of document properties as metadata. But in some contexts it is natural to think of them as data for the entity described by the document.