SpamAssassin Rule: TVD_RCVD_SINGLE
Standard description: Unfortunately I can find no descriptor for this rule in my searching thus far.
Based on my limited knowledge of Perl, it appears from this line "Received =~ /from\s+(?!localhost)[\s.a-z0-9-]+\s/" that the TVD_RCVD_SINGLE header is used when a "Received" line in the SMTP Transmission header contains "localhost" as a server name.
The default scores for this rule can be found in the online list of tests. At the time of this writing the default score was +1.351 (see share/spamassassin/72_active.cf).