docs: add ISO 4217 currency code guidance for TIP-20 issuers

src/pages/guide/issuance/create-a-stablecoin.mdx

@@ -151,6 +151,10 @@ Now that we have some input fields, we need to add some logic to handle the subm
 
 After this step, your users will be able to create a stablecoin by clicking the "Create" button!
 
+:::warning
+We **strongly** recommend that for stablecoins, the `currency` field be set to the [ISO 4217](https://www.iso.org/iso-4217-currency-codes.html) three-letter code for the underlying fiat currency (e.g., `"USD"`, `"EUR"`, `"GBP"`) — **not** the token symbol. This value is **immutable** after token creation and affects fee payment eligibility, DEX routing, and quote token pairing. **Only `USD` stablecoins can be used to pay transaction fees on Tempo** — if your stablecoin is USD-denominated, the currency must be set to `"USD"` to be eligible for fee payment.
+:::
+
 <Demo.Container name="Create Form">
   <Step.CreateToken stepNumber={1} last />
 </Demo.Container>

src/pages/protocol/tip20/overview.mdx

@@ -130,7 +130,32 @@ TIP-20 supports an opt-in [reward distribution system](/protocol/tip20-rewards/o
 
 ### Currency Declaration
 
-A TIP-20 token can declare a currency identifier (e.g., `"USD"`, `"EUR"`) that identifies the real-world asset backing the token. This enables proper routing and pricing in Tempo's [Stablecoin DEX](/protocol/exchange). USD-denominated TIP-20 tokens can be used to pay transaction fees and serve as quote tokens in the DEX.
+A TIP-20 token can declare a currency identifier (e.g., `"USD"`, `"EUR"`) that identifies the real-world asset backing the token. This enables proper routing and pricing in Tempo's [Stablecoin DEX](/protocol/exchange). **Only `USD`-denominated stablecoins can be used to pay transaction fees on Tempo.** USD-denominated TIP-20 tokens also serve as quote tokens in the DEX.
+
+Stablecoin currency identifiers should be [ISO 4217](https://www.iso.org/iso-4217-currency-codes.html) three-letter currency codes. Common examples include:
+
+| Code  | Currency               |
+|-------|------------------------|
+| `USD` | United States Dollar   |
+| `EUR` | Euro                   |
+| `GBP` | British Pound Sterling |
+| `JPY` | Japanese Yen           |
+| `SGD` | Singapore Dollar       |
+| `BRL` | Brazilian Real         |
+| `AED` | UAE Dirham             |
+
+The `currency` field represents the **denomination**, not the token itself — multiple tokens can share the same currency code:
+
+| Token         | Correct `currency` | Incorrect `currency` |
+|---------------|---------------------|----------------------|
+| USDC          | `"USD"`          | `"USDC"`           |
+| USDT          | `"USD"`          | `"USDT"`          |
+| PYUSD         | `"USD"`          | `"PYUSD"`          |
+| EURC          | `"EUR"`          | `"EURC"`           |
+
+:::warning
+The currency code is **immutable** — it cannot be changed after token creation. An incorrect currency code will affect fee payment eligibility, DEX routing, and quote token pairing.
+:::
 
 ### DEX Quote Tokens
 

src/pages/protocol/tip20/spec.mdx

@@ -411,7 +411,7 @@ TIP-20 tokens cannot be sent to other TIP-20 token contract addresses. The imple
 Any attempt to transfer to a TIP-20 token address must revert with `InvalidRecipient`. This prevents accidental token loss by sending funds to token contracts instead of user accounts.
 
 ## Currencies and Quote Tokens
-Each TIP-20 token declares a currency identifier and a corresponding `quoteToken` used for pricing and routing in the Stablecoin DEX. Tokens with `currency == "USD"` must pair with a USD-denominated TIP-20 token.
+Each TIP-20 token declares a currency identifier and a corresponding `quoteToken` used for pricing and routing in the Stablecoin DEX. Stablecoin currency identifiers should be [ISO 4217](https://www.iso.org/iso-4217-currency-codes.html) three-letter codes representing the underlying fiat currency (e.g., `"USD"`, `"EUR"`, `"GBP"`) — not the token's own symbol. The currency is set at token creation and **cannot be changed afterward**. **Only tokens with `currency == "USD"` are eligible for paying transaction fees.** Tokens with `currency == "USD"` must pair with a USD-denominated TIP-20 token.
 
 Updating the quote token occurs in two phases:
 1. `setNextQuoteToken` stages a new quote token.
@@ -470,7 +470,7 @@ interface ITIP20Factory {
     /// @notice Creates and deploys a new TIP-20 token
     /// @param name The token's ERC-20 name
     /// @param symbol The token's ERC-20 symbol
-    /// @param currency The token's currency identifier (e.g. "USD")
+    /// @param currency The token's ISO 4217 currency code (e.g. "USD", "EUR"). Immutable after creation.
     /// @param quoteToken The TIP-20 quote token used for exchange pricing
     /// @param admin The address to receive DEFAULT_ADMIN_ROLE on the new token
     /// @param salt A unique salt for deterministic address derivation