The SPF Record uses Qualifiers and Arguments for each Mechanism to describe the hosts which are designated as proper sources for SMTP traffic for the domain.
Qualifiers
The Qualifier defines how the source should be treated. The default, if none is given, is to give the Mechanism a Pass.
Pass: Host allowed to send (Accept)
Fail: Host NOT allowed to send (Reject)
SoftFail: Host not allowed to send, yet may be in transition (Accept, Mark)
Neutral: Explicitly nothing is said about the validity (Accept)
None: Host results are not evaluated (Accept)
Mechanisms
Defines the DNS Source to examine for reference.
All other source items. Usually found at the end. Example:~all
An IPv4 address or network range definition (in CIDR format). /32 is assumed if no /<network-length> is defined. Example:ip4:192.168.1.254
An IPv6 address or network range definition. /128 is assumed if no /<network-length> is defined. Example:ip6:fdc0:a8:ff:1::1
Look for client IP(s) in the A Records of <domain>. Example:+a
Look for the client IP(s) in the MX Records of <domain>. Example:~mx
Hostname of the client IP is queried by PTR and validated by the A Records of <domain>. Example:ptr
Query like A Record, yet any match is accepted.
<domain> is searched for a match. Example:include:spf.example.com