Follows every include and counts the DNS lookups against the limit of ten.
SPF lists the servers allowed to send mail for a domain. It has a hard limit that catches most growing companies: evaluating the record may cost no more than ten DNS lookups, counting everything inside every include. One more sending service and SPF stops working for the whole domain, including the senders already listed.
This check follows the record the way a receiving server does and shows the full tree, so you can see which include is spending the budget.
Receivers return a permanent error and treat SPF as failed or ignore it, so legitimate mail from the senders you listed can be rejected or land in spam.
Remove includes for services you no longer use, replace a: and mx: with ip4: ranges, and ask whether each marketing tool needs to send as the main domain or could use a subdomain.
No. Exactly one record is allowed. Two are treated as an error and SPF fails. Merge them into one.
More than two includes or mechanisms that return nothing is a permanent error under the standard. It usually means a service was cancelled and its include left behind.