When using FreeDNS configuration they always have acronyms for the different types of records. Below is what they all mean.
Type: A – Point subdomain.domain.com to a hard coded IP Address. Most direct and straight forward option, also note any change you make in the FreeDNS program is reflected on the internet and made live immediately. The only way you will not see immediate results is if you have cached a query on your computer by looking it up PRIOR to configuring it in the FreeDNS program.
Type: MX – Point subdomain.domain.com to a mail server. These type of records are special for just mail servers, they can co-exist with A records, and their only use is for routing mail to a different location. All mail implementations check for this record first before attempting to route an e-mail message. If a MX record does not exist for a host, an e-mail delivery would be attempted directly to the IP that the hostname resolves to.
Type: AAAA – Point subdomain.domain.com to a IPv6 address. Useful for those who are using IPv6 on their personal networks or those who are using a IPv4 to IPv6 tunnel at home.
Type: CNAME – Point subdomain.domain.com to another hostname. Good for those who are using other dynamic DNS services. You can create a CNAME record to another host and whatever subdomain.domain.com you choose here will go to whatever IP address the CNAMEd host has.
Type: NS – Point subdomain.domain.com to another NAMESERVER. If you choose this option, then whatever subdomain.domain.com address you choose using FreeDNS will have to be configured and setup on the destination ADDRESS (nameserver) that you choose. This option basically means you are delegating a FreeDNS host to another DNS server all together, so when you choose this option you are telling every computer on the internet to ask the ‘address’ where subdomain.domain.com is located at. If the host you point an NS record to is not configured to answer for the subdomain.domain.com that you are using in FreeDNS then the subdomain.domain.com host will not resolve.
Type: TXT – Lets you create TXT records, used for a number of different things, most commonly for DKIM records (for combatting spam) so other receiving mail servers can verify email was sent from you by verifying your publically published crypto-signature. Wrap your TXT “destination” in quotes (don’t worry, the system will remind you if you forget).
Type: SPF – A anti-spam record, good to have on any domain you’re sending email with. See https://www.spfwizard.net/ for more details.