documentation: Blueprints extends and import

[drzraf]

I'm trying to extends a form and tend to think that documentation is sparse.
extends and import deserve much more than what provides these two documentation sections.
Even the parent@ is a bit hard to catch with. Where is the inheritance defined?

Digging the sourcecode does help as expected. For example blueprints documentation at https://learn.getgrav.org/forms/blueprints is not sufficient to understand BlueprintForm.php,

An overview of the (YAML) functions available, the file path they are looking for, the contexts where they can be used and the YAML structures merge strategies, the level at which they can be used would really help in order to use these constructs at there full value.
Eg: can I import a form:? its ""items""? rules included? or only its fields: and what about extends?