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In a federated graph, different subgraphs may define the same field on different concrete union member types with differing nullability. For example, one subgraph might define upc: ID! on Cosmo while another defines upc: ID (nullable) on Consultancy, both members of a Products union. By default, the router rejects queries that select such fields across union members with a validation error like: 
fields 'upc' conflict because they return conflicting types 'ID' and 'ID!'
The relax_subgraph_operation_field_selection_merging_nullability option relaxes this validation when the enclosing types are non-overlapping concrete object types (i.e., different union members). This is useful when subgraphs legitimately disagree on nullability for the same field.

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Examples

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Scalar fields with differing nullability

Given two union member types where upc has different nullability: 
# Subgraph A
type Cosmo {
  upc: ID!
}

# Subgraph B
type Consultancy {
  upc: ID  # nullable
}

union Products = Cosmo | Consultancy
The following query selects upc on both types: 
{
  products {
    ... on Consultancy { upc }
    ... on Cosmo { upc }
  }
}
Default behavior: The router rejects this with a field conflict error. With relaxed nullability enabled: The query is accepted because Consultancy and Cosmo are non-overlapping concrete types — a result can only ever be one or the other.

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Object fields with differing nullability

The same applies to object-typed fields: 
# Subgraph A
type Cosmo {
  lead: Employee!
}

# Subgraph B
type Consultancy {
  lead: Employee  # nullable
}

{
  products {
    ... on Consultancy { lead { id } }
    ... on Cosmo { lead { id } }
  }
}
Default behavior: The router rejects this with a type conflict error. With relaxed nullability enabled: The query is accepted.

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Configuration

ENGINE_RELAX_SUBGRAPH_OPERATION_FIELD_SELECTION_MERGING_NULLABILITY=true
This option is disabled by default. It only affects validation of field selections where the enclosing inline fragment types are non-overlapping concrete union members. See the Router Configuration reference for all engine flags.