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.

GET /shops/2/products/31

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.