include, a, mx, ptr, exists, and redirect.
include:domain
Include another domain's SPF (DNS lookup)
ip4:address/prefix
Authorize IPv4 address or range
ip6:address/prefix
Authorize IPv6 address or range
a[:domain]
Authorize A records (DNS lookup)
mx[:domain]
Authorize MX records (DNS lookup)
all
Match everything (usually last)
exists:domain
Check if domain resolves (DNS lookup)
redirect=domain
Use another domain's SPF instead