XLS-52: NFTokenMintOffer Specification (#224)

XLS-0052-NFTokenMintOffer/README.md

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+<pre>
+title: NFTokenMintOffer
+type: Spec
+author: tequ
+</pre>
+
+## Abstract
+
+This proposal extends the minting capabilities of NFToken (XLS-20).
+
+NFToken are not only held by the issuer, but are often distributed or sold by the issuer to others.
+
+XRPL NFToken requires two transactions (`NFTokenMint` and `NFTokenCreateOffer`) to be sent by the issuer before the NFToken can be minted and distributed (or sold).
+NFTokenOfferMint extends the existing `NFTokenMint` transaction to allow an **NFToken Sell Offer** to be created at the same time as the NFToken is minted.
+
+NFTokenOfferMint is expected to significantly improve the experience of NFT projects and users.
+
+## Specification
+
+### New `NFTokenMint` Transaction Field
+
+Add 3 new fields to the`NFTokenMint` transaction.
+
+| Field Name    | Required? | JSON Type            | Internal Type |
+|-------------- |:---------:|:-------------------: |:-------------:|
+| `Amount`      |           | `Currency Amount`    | `AMOUNT`      |
+| `Destination` |           | `string`             | `AccountID`   |
+| `Expiration`  |           | `number`             | `UINT32`      |
+
+These fields have the same meaning as the field of the same name used in the `NFTokenCreateOffer` transaction, but the `Amount` field is not a required field.
+
+If the `Amount` field (and the other 2 fields) are not specified, `NFTokenMint` transaction behaves exactly like a previous `NFTokenMint` transaction.
+
+### Creating an `NFTokenOffer`
+
+Since NFToken issuers can only create a sell offer for their NFToken, the `Owner` field is not set and `tfSellNFToken` flag is always set in the `NFTokenOffer` created in an `NFTokenMint` transaction.
+
+In an extended `NFTokenMint` transaction, an `NFTokenOffer` is created when
+
+- `Amount` field is specified.
+
+An error occurs in the following cases
+
+- One or both of `Destination` and `Expiration` fields are specified, but the `Amount` field is not .
+
+### Owner Reserve
+
+The `NFTokenPage` and `NFTokenOffer` reserves will not change, but will create the possibility of up to 2 incremental reserves (NFTokenPage, NFTokenOffer) in an `NFTokenMint` transaction.