How to use "iconClass" prop in a BalmUI "floating action button" component?

I started a project with BalmUI using vue cli. In one of the components that This framework has (I mean floating action button), The docs says that we can use a prop called iconClass in the slots of this component. The example in the docs is this one:

<ui-fab>
  <template #default="{ iconClass }">
    <svg-logo :class="iconClass"></svg-logo>
  </template>
</ui-fab> 

But it does not say anything else about this prop. Is this a icon name of material icons, or a class that we can add custom styles to the slot or anything else. I tried to understand the meaning of that prop by creating two vue components. The parent component is called HelloWorld.vue has these codes:

> HelloWorld.vue:

<template>
<div class="hello">

    <ui-fab>
     <template #default="{ iconClass }">
      <svg-logo :class="iconClass"></svg-logo>
     </template>
    </ui-fab>

</div>
</template>

<script>
import SvgLogo from '../components/SvgLogo.vue';

export default {
  name: 'HelloWorld',
  data() {
    return {
        iconClass: {
            myClass: true
        }
    }
  }, // end of data
    components: {
        SvgLogo
    },
}
</script>

<!-- Add "scoped" attribute to limit CSS to this component only -->
<style lang="scss">
    
    .myClass {
        color: red;
        font-size: 50px;
    }
    
    .iconClass {
        color: red;
        font-size: 50px;
    }
    
</style>

And the second component has these codes:

> SvgLogo.vue:

<template>
   <div>
       HD
   </div>
</template>

<style scoped>
    
    .myClass {
        color: red;
        font-size: 50px;
    }
    
    .iconClass {
        color: red;
        font-size: 50px;
    }
    
</style>

But nothing happens in my result page. I mean that no style is applied to the SvgLogo component. I could not understand what is wrong in my codes?

After some search in my app and using browser devtools, I found that this prop and also some other similar props in other balmui components slots, works internally. I mean that they don't need any class or vue data from us. If we omit the iconClass for example from that code:

<ui-fab>
  <template #default>
    <svg-logo></svg-logo>
  </template>
</ui-fab>

Then in the browser devtools, we can see something like this image for the fab btn:

If we notice to the "HD" word in that part (that comes from svg-logo component), there is no class added to that. On the other hand with using iconClass prop:

<ui-fab>
  <template #default="{ iconClass }">
    <svg-logo :class="iconClass"></svg-logo>
  </template>
</ui-fab> 

We could see something like this image in the browser devtools:

In this case the mdc-fab__icon class is added to that div containing HD word. So the balmui itself added some styles to that div like these:

Because the docs did not say anything about this prop, I thought that it is a class name or a data that we must add in our code to customize the fab component, But I was wrong about that.