It was recently accepted for standardization by nist There is a unique fors tree below each of the $2^ {63}$ level 0 xmss leaf nodes That is, each leaf node signs a different fors public key (and each node only signs its assigned public key). Now, as we generate bitmasks pseudorandomly, this has no influence on public key size A fors key pair does not consist of a single Monolithic tree but of k trees of height log t.
Here is an example of a small tree from the sphincs+ paper and a diagram of one fts node.
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