The role of A type records consists in mapping a name to one or more IP addresses. The assumption here is that the IP address is known and stable. In your screen capture, the A record row always indicates an IP address under the Data column. The specific restriction is related to IP addresses, not subdomains. For related detail, you may have a look at the "Using subdomains" sub-chapter <https://cloud.google.com/appengine/docs/standard/python/mapping-custom-domains#using_subdomains> of the "Mapping Custom Domains" online document.
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