BIP-118 defines two ANYPREVOUT variants that differ in how much of the previous output they omit from the digest, summarised in Figure 2.
Under SIGHASH_ANYPREVOUT, the outpoint is excluded from the digest, but the signature still commits to the amount and scriptPubKey of the previous output, as well as the input’s nSequence
Under SIGHASH_ANYPREVOUTANYSCRIPT, the amount and scriptPubKey are also excluded, meaning the signature is not bound to the locking script of the spent output at all. All other commitments follow the standard Taproot signature message construction and depend on the selected base flag, such as SIGHASH_ALL or SIGHASH_SINGLE. Because the outpoint is omitted from the digest, the same signature can authorize spending any compatible UTXO that satisfies the remaining committed fields.
For example, a transaction pre-signed with ANYPREVOUT | ALL to produce a 0.5 BTC output can be reused if the same address later receives another UTXO of 0.5 BTC, even if the private key used to create the original signature is no longer available. If the new UTXO holds more than 0.5 BTC, however, the excess will be lost to miners unless the original signature included a change output. This rebinding property is what makes ANYPREVOUT useful for layer-2 protocols, where the same pre-signed transaction must apply to multiple possible on-chain UTXOs without requiring new signatures for each one.