Short answers, pulled from the story.
The numerus clausus is a property law concept that limits the types of rights courts will recognise as having the character of "property." It holds that individuals cannot freely create new categories of property rights; recognition must come from the courts. The principle traces its roots to Roman law and holds especial importance in land law and corporate law.
Under English law today there are fourteen recognised property rights in the numerus clausus. They include freehold ownership, easements, restrictive covenants, leases, mortgages, rights under trusts, licenses coupled with an interest, profit a prendre, rentcharges, rights of entry, estate contracts, options and pre-emption rights, mere equities, and home rights. Before the Law of Property Act 1925, the life estate and the entail also counted, but they were abolished as freestanding property rights.
In National Provincial Bank Ltd v Ainsworth, Lord Wilberforce stated that before a right can be admitted into the category of property it must be definable, identifiable by third parties, capable in its nature of assumption by third parties, and must have some degree of permanence or stability. This formulation came in the twentieth century and represented a more open approach than earlier nineteenth-century rulings.
Holders of property rights receive privileged remedies that personal right holders do not. These include priority in payment from an insolvent debtor, a greater likelihood of being awarded specific performance, and security in remaining in possession of land or another asset. A personal right such as a contract or obligation to pay compensation does not carry these advantages.
German law gives the numerus clausus a constitutional foundation and applies it along two axes: Typenzwang limits the number of permissible property rights, and Typenfixierung fixes the content of each type so parties cannot alter its essential character by agreement. This makes the German version more rigid than the English approach, which relies on judicial recognition through case law.
France and Spain are the notable exceptions among European states that do not follow equal numerus clausus doctrines. Many other European jurisdictions apply comparable principles, making France and Spain stand apart from the broader continental trend.