Why clipplanes are computed in eye space?

CesiumJS
1

In ModelClippingPlanesStageFS, CesiumJS use clip function to compute the fragment will be display or not:

float clip(vec4 fragCoord, sampler2D clippingPlanes, mat4 clippingPlanesMatrix) {
    vec4 position = czm_windowToEyeCoordinates(fragCoord);
    vec3 clipNormal = vec3(0.0);
    vec3 clipPosition = vec3(0.0);
    float pixelWidth = czm_metersPerPixel(position);
    
    #ifdef UNION_CLIPPING_REGIONS
    float clipAmount; // For union planes, we want to get the min distance. So we set the initial value to the first plane distance in the loop below.
    #else
    float clipAmount = 0.0;
    bool clipped = true;
    #endif

    for (int i = 0; i < CLIPPING_PLANES_LENGTH; ++i) {
        vec4 clippingPlane = getClippingPlane(clippingPlanes, i, clippingPlanesMatrix);
        clipNormal = clippingPlane.xyz;
        clipPosition = -clippingPlane.w * clipNormal;
        float amount = dot(clipNormal, (position.xyz - clipPosition)) / pixelWidth;

        #ifdef UNION_CLIPPING_REGIONS
        clipAmount = czm_branchFreeTernary(i == 0, amount, min(amount, clipAmount));
        if (amount <= 0.0) {
            discard;
        }
        #else
        clipAmount = max(amount, clipAmount);
        clipped = clipped && (amount <= 0.0);
        #endif
    }

    #ifndef UNION_CLIPPING_REGIONS
    if (clipped) {
        discard;
    }
    #endif

    return clipAmount;
}

My question is why is the clipNormal and position that computes amout under eyeSpace:

vec4 clippingPlane = getClippingPlane(clippingPlanes, i, clippingPlanesMatrix);
clipNormal = clippingPlane.xyz;
clipPosition = -clippingPlane.w * clipNormal;
float amount = dot(clipNormal, (position.xyz - clipPosition)) / pixelWidth;

Is there any reason for this?