From HCE by Mirko Buffini

Every perfume is compared on up to five signals. Community accord votes weigh the most, followed by notes, where rare notes count more than common ones like musk. Note families, seasons and the men, women or unisex leaning make up the rest.

The brand is never used, so a match from another house is as likely as one from the same house. When a perfume has no data for a signal, that signal is skipped instead of counted as zero.