What does the cardinality (the defenition tab) in a SECTION stand for?
It regulates the “items” attribute of a SECTION.
Is is how many times the SECTION can occur in one COMPOSITION?
No, it’s not. If you want to achieve this, you need to specify the cardinality of “content” attribute of COMPOSITION.
In the tree-structure each subsection or slot is constrained with occurences. But is there an overall cardinality for the whole SECTION?
The overall cardinality for the whole SECTION should appear in a container type attribute of a class that can possibly include instances of SECTION. In openEHR, it is either COMPOSITION or SECTION.