element 復選框組件 想知道這段代碼是什么意思？

emrParts = ['入院記錄', '病程記錄', '出院小結', '手術記錄'];
value是從復選框傳過來的參數(shù)
 const checkedCount = value.length;
 this.checkAll = checkedCount === this.emrParts.length;
 this.isIndeterminate = checkedCount > 0 && checkedCount < this.emrParts.length;
element標準寫法:
<template>
  <el-checkbox :indeterminate="isIndeterminate" v-model="checkAll" @change="handleCheckAllChange">全選</el-checkbox>
  <div style="margin: 15px 0;"></div>
  <el-checkbox-group v-model="checkedCities" @change="handleCheckedCitiesChange">
    <el-checkbox v-for="city in cities" :label="city" :key="city">{{city}}</el-checkbox>
  </el-checkbox-group>
</template>
<script>
  const cityOptions = ['上海', '北京', '廣州', '深圳'];
  export default {
    data() {
      return {
        checkAll: false,
        checkedCities: ['上海', '北京'],
        cities: cityOptions,
        isIndeterminate: true
      };
    },
    methods: {
      handleCheckAllChange(val) {
        this.checkedCities = val ? cityOptions : [];
        this.isIndeterminate = false;
      },
      handleCheckedCitiesChange(value) {
        let checkedCount = value.length;
        this.checkAll = checkedCount === this.cities.length;
        this.isIndeterminate = checkedCount > 0 && checkedCount < this.cities.length;
      }
    }
  };
</script>
想知道handleCheckedCitiesChange這個方法中的代碼什么意思?