This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
mode = ConcurrentMode;
    if (isStrictMode === true) {
      mode |= StrictLegacyMode;
      if (enableStrictEffects) {
        mode |= StrictEffectsMode;
      }
    } else if (enableStrictEffects && createRootStrictEffectsByDefault) {
      mode |= StrictLegacyMode | StrictEffectsMode;
    }
    if (
      
      
      !enableSyncDefaultUpdates ||
      
      (allowConcurrentByDefault && concurrentUpdatesByDefaultOverride)
    ) {
      mode |= ConcurrentUpdatesByDefaultMode;
    }
  } else {
    mode = NoMode;
  }
  if (enableProfilerTimer && isDevToolsPresent) {
    
    
     
    mode |= ProfileMode;
  }
  return createFiber(HostRoot, null, null, mode);
}
export function createFiberFromTypeAndProps(