What are SPF mechanisms (e.g., ip4, a, mx, include)?

Mechanisms define which servers are authorized. The main ones are:

ip4 authorizes a specific IPv4 address or range.

a authorizes the IPs of the domain’s A record.

mx authorizes the IPs of the domain’s MX records.

include imports the SPF policy of another domain.

Mechanisms are the items on the ship registry showing which vessels carry the domain’s colors.