What does underscore mean in spf entry?

Question

SPF

What does underscore mean in spf entry?
Here is an example:

v=spf1 include:_spf.google.com include:spf.mandrillapp.com -all

Answer 1

it's not special. character, and part of fqdn.

id 29830
opcode QUERY
rcode NOERROR
flags QR RD RA
;QUESTION
_spf.google.com. IN TXT
;ANSWER
_spf.google.com. 299 IN TXT "v=spf1 include:_netblocks.google.com include:_netblocks2.google.com include:_netblocks3.google.com ~all"
;AUTHORITY
;ADDITIONAL

Answer 2

Inserting include:_spf.google.com means that the number of IP addresses allowed for sending emails from your domain must include the IP addresses specified in the TXT record of the _spf subdomain of the google.com domain
The underscore character is often used in email TXT records. For example, DMARC is registered for the _dmarc subdomain of the domain, DKIM is registered through the _domainkey subdomain with the addition of a selector, for example, the standard DKIM for Google WorkSpace is registered through the google._domainkey subdomain, for Yandex Connect - through mail._domainkey, for Mile ru for business - through
mailru._domainkey I'm not sure, but it seems to me that this was done in order to separate mail records from records of subdomains of the site