- * Registers interest in spends of a transaction output identified by `outpoint` having
- * `script_pubkey` as the spending condition.
+ * Registers interest in spends of a transaction output.
+ *
+ * Optionally, when `output.block_hash` is set, should return any transaction spending the
+ * output that is found in the corresponding block along with its index.
+ *
+ * This return value is useful for Electrum clients in order to supply in-block descendant
+ * transactions which otherwise were not included. This is not necessary for other clients if
+ * such descendant transactions were already included (e.g., when a BIP 157 client provides the
+ * full block).