ReflectionProbe.cullingMask

Description

This is used to render parts of the reflecion probe's surrounding selectively.

If the GameObject's layerMask AND the probe's cullingMask is zero then the game object will be invisible from this probe. See Layers for more information.

using UnityEngine;
using UnityEditor;

public class ExampleScript : MonoBehaviour
{
    void Start()
    {
        var probe = GetComponent<ReflectionProbe>();
        // Only render objects in the first layer (Default layer) when capturing the probe's texture
        probe.cullingMask = 1 << 0;
    }
}

Additional resources: Camera.cullingMask.