The use of the question mark in JSON specifies that if the field being referenced doesn't exist then a NULL value will be returned instead of throwing an error. One of the common spots where you won't see the ? is when you are referencing a value that is part of an array. For example
item()?['some field']['1']?['value field']
In this example 'some field' may or may not be returned as part of a JSON object. If its not returned then a null will be returned if you try to use the value of the field. But the field is supposed to be part of an array and we want the second element in the array. In the case of the index the value either exists or its an invalid array index, so we return an error rather than a null.
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