mask-type

html,
body,
svg {
  height: 100%;
}

<svg viewBox="0 0 100 100" xmlns="http://www.w3.org/2000/svg">
  <!-- Two identical masks other than the id and mask-type values -->
  <mask id="myMask1" mask-type="alpha">
    <rect
      fill="rgb(10% 10% 10% / 0.4)"
      x="0"
      y="0"
      width="100%"
      height="100%" />
    <circle fill="rgb(90% 90% 90% / 0.6)" cx="50" cy="50" r="35" />
  </mask>

  <mask id="myMask2" mask-type="luminance">
    <rect
      fill="rgb(10% 10% 10% / 0.4)"
      x="0"
      y="0"
      width="100%"
      height="100%" />
    <circle fill="rgb(90% 90% 90% / 0.6)" cx="50" cy="50" r="35" />
  </mask>

  <!-- The first rect is masked with an alpha mask -->
  <rect x="0" y="0" width="45" height="45" mask="url(#myMask1)" fill="red" />

  <!-- The last rect is masked with a luminance mask -->
  <rect x="55" y="0" width="45" height="45" mask="url(#myMask2)" fill="red" />
</svg>