What is the difference between a "meta model" and "design patterns"?
Design Pattern: It is a "good practice". It can be considered as a possible solution for a problem that may ofter occur. The solution usually comes from the experience.  Examples are MVC and Identify Field.

Metamodel: It can be considered as a set of rules, constraints and constructs that allow to model a problem.
As an example you can consider a map.



A map allows to model the reality, so a map can be considered the model. The metamodel of a map is the legend: the constructs that you need to use in order to represent borders, rivers, lakes,...

In your case Identify Field is just a good practice that you can decide to follow or not. It's not a constraint/rule.