Perhaps they are referring to the syntactic sugar around keyword lists?

[a: 1, b: 2] == [{:a, 1}, {:b, 2}]

Or maybe atom vs string keys in maps?

%{a: 1} vs %{"b" => 1}

Or keyword lists always needing to come last in lists?

[some: :value, :another] # error

[:another, some: :value] # valid

Or maybe something else entirely. Those are just things I remember having to lookup repeatedly when I was first learning elixir.