Records for unmapped query sequences are discarded unless the -output_no_hits option is specified.
If the SAM records contain MD tags, or if the original search database is specified using the -db option, then the alignments in the records are validated for consistency with the target sequences. This can catch bugs in the CIGAR string or MD tag, which are surprisingly common in popular software that supports SAM.
Human-readable alignments can be written to a file specified by the -alnout option. This requires that MD tags are present in the SAM records or that the -db option is specified.
Alignments in BLAST 6 format can be written to a file specified by the -blast6out option.
E-values are calculated for the output files, which requires that the database size is known. If a database file is given, the size of the database is used to calculate E-values. Otherwise, the ka_dbsize option can be used to specify the database size in letters.
usearch -sam_filter hits.sam -db genome.fa -alnout hits.aln