No brother. 18 is not pointing to a property.
The explanation for the 4th rule
lets say we have category object like this
name: "Category Name",
price : 12
It says don't point to the price. If you want only the price then add query parameter as described in rule 14.
The 18th rule says use relation.
this URL means if you want to get product with ID 31 from Shop with ID 2. you should use this structure.
I hope you understand. pardon me if I failed to explain it properly.