Distance accessory binary genes

[EllistonV]

Hello,

I only have a quick question. If this tree is based on the binary presence and absence of accessory genes, what does the distance in the tree means? I made a tree with Figtree, as suggested in the documentation, but how is the distance calculated. If the distance is calculated through an MSA, but this is not using an alignment, only the presence or absence of genes, then what does the distance means?