The last record you need to know is the SOA record, which is a mandatory record in all zone files, and is used to describe the zone and configure how often nameservers should update the zone file for this domain. It also has a version counter that ensures that only the latest version of the zone file is propagated:

hobnob.social.  IN  SOA  ns1.digitalocean.com.  dan.danyll.com  ( <serial>, <refresh>, <retry>, <expiry>, <negativeTTL> )

The first few values are similar to the ones in the NS records. The rest are as follows:

• ns1.digitalocean.com is the primary master nameserver, which holds the most up-to-date zone file. There may be slave nameservers that mirror the primary nameserver to reduce its load.
• dan.danyll.com is the email for the administrator responsible for this DNS zone. The @ symbol has been replaced by a period (.); if you have a period in your email address, it would be replaced by a backslash (\).
• <serial> is the serial number for the zone file, which is essentially a version counter. Every time your zone is updated, you should also increase the serial number by 1. Slave nameservers will check this serial number to determine whether their own zone file is outdated.
• <refresh> is the amount of time a slave nameserver will wait before pinging the master server to see whether it needs to update its zone file.
• <retry> is the amount of time a slave nameserver will wait before pinging the master server again, if the previous connection attempt was unsuccessful.
• <expiry> is the amount of time that the zone file should still be deemed to be valid, even if it was no longer able to connect to the master server to update it.
• <negativeTTL> is the amount of time the nameserver will cache a lookup that failed.

Again, since these values don't need to change often, and because having to manually update the serial number every time we update our zone file is tedious and error-prone, DigitalOcean has preset and hidden these values for us. DigitalOcean will update our SOA record for us when we update our records using DigitalOcean's web console.

Now, just make sure you have the A record set for the api.hobnob.social subdomain and move on to the next section.