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GraphQL supports two kinds of abstract types -- unions and interfaces. An abstract type is a type that represents two or more possible types. Abstract types allow you to specify a single type for your field that could be one of several possible types at runtime (i.e. when the query is executed). When executing a query, GraphQL can never return an abstract type -- instead, the type has to be resolved into one of the possible types when the query is executed.

If a field returns a list, then the type for each item in the list will resolved separately. This type resolution happens before any of the fields on each item are resolved. More to the point, the type that's resolved determines which fields need to be resolved in the first place.

In your example above, you've defined an abstract type (the interface Round) and several possible types for it (LightningRound, MultipleChoiceRound, etc.). However, you have not told GraphQL how to determine whether a Round is a LightningRound, a MultipleChoiceRound or another possible type. This is the purpose of providing a resolveType function. You typically define a resolveType function for each abstract type in your schema. Assuming you're using graphql-tools or apollo-server, you provide this function through the same resolver map object you use to define your resolvers: