The optional metafields on carrier service rate responses can be accessed on the rate’s associated delivery option at checkout.
Note: presently, these metafields are not persisted beyond their association to delivery options past checkout.
The metafields can be read in Checkout UI Extensions on the Shipping Option from the Delivery API.
The metafields on carrier service rates are modelled like metafields on delivery option function results: they are not persisted past checkout and used to inform logic in Checkout UI Extensions.