• array[index]: specifies the element at the specified index in the array.
• array[FIRST]: secifies the first element in the array.
• array[LAST]: specifies the last element in the array.
• array[SIZE]: specifies the size of the array.

Let’s say we have a person with many dogs. index should be replaced with a number which will return the dog that you want to check against. Here we’re checking if the first dog’s age is 5.
dogs[0].age = 5
Here we’re checking if a person has 3 dogs
dogs[SIZE] = 3

Part of https://github.com/realm/realm-core/issues/2978