What is an A record?
An A record maps a host name to an IPv4 address, the number a browser or mail server actually connects to. A name can hold several A records, and senders and browsers pick among them. When the name is an alias (a CNAME), resolvers follow it and return the A records of the name it points to.
Reading the result
The addresses come with their TTLs, after any CNAME chain that led to them. An address in a private or reserved block is named with the RFC that reserves it, because a public record pointing there sends everyone somewhere they cannot reach. If Google and Cloudflare return different addresses, both lists are shown: services that answer by location do that on purpose.
Beat runs on the same engine, on a schedule, across all 41 checks and 25 blacklists. What it adds is asking again, noticing when an answer changes and telling you what to do about it.
Common problems it finds
A private address in public DNS
Addresses in 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16 (RFC 1918) exist only inside one network. Published publicly, they usually come from an internal record copied into the wrong zone.
An alias that leads nowhere
If the CNAME chain ends at a name that does not exist, the lookup returns NXDOMAIN and nothing can connect. The CNAME checker follows the chain hop by hop.
A long TTL before a move
Resolvers keep an address for its full TTL. Lower the TTL a day ahead of changing servers, or some visitors and senders keep reaching the old address until it expires.