Tree shows an X at the end of branches... what do they mean?

Hi there, this may be a silly question, but what do the black X at the end of tree branches mean?

It means it was selected/used as a vaccine strain. There’s a little more info when you hover over the tip (see screenshot below). This functionality has been around for a long time but we haven’t really developed on it much, and (to my knowledge) seasonal-flu datasets are the only ones we use it for.

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Thank you for a quick response! This was showing up for me using a PRRS dataset!