{
  "$schema": "https://developer.microsoft.com/json-schemas/sp/v2/column-formatting.schema.json",
  "elmType": "div",
  "children": [
    {
      "forEach": "choiceIterator in @currentField",
      "elmType": "div",
      "txtContent": "[$choiceIterator]",
      "attributes": {
        "class": "ms-bgColor-themePrimary ms-fontColor-white",
        "title": "='I am the letter ' + [$choiceIterator]"
      },
      "style": {
        "width": "16px",
        "height": "16px",
        "text-align": "center",
        "margin": "1px"
      }
    }
  ]
}