UID2 module: Add documentation for cstg

modules/uid2IdSystem.md

@@ -1,11 +1,46 @@
 ## UID2 User ID Submodule
 
-UID2 requires initial tokens to be generated server-side. The UID2 module handles storing, providing, and optionally refreshing them. The module can operate in one of two different modes: *Client Refresh* mode or *Server Only* mode.
+The UID2 module handles storing, providing, and optionally refreshing tokens. While initial tokens traditionally required server-side generation, the introduction of the *Client-Side Token Generation (CSTG)* mode offers publishers the flexibility to generate UID2 tokens directly from the module, eliminating this need. Publishers can choose to operate the module in one of three distinct modes: *Client Refresh* mode, *Server Only* mode and *Client-Side Token Generation* mode.
 
 *Server Only* mode was originally referred to as *legacy mode*, but it is a popular mode for new integrations where publishers prefer to handle token refresh server-side.
 
+*Client-Side Token Generation* mode is included in UID2 module by default. However, it may not be necessary for all publishers. You can instruct the build to exclude code for this feature:
+
+```
+    $ gulp build --modules=uid2IdSystem --disable UID2_CSTG
+```
+
 **Important information:** UID2 is not designed to be used where GDPR applies. The module checks the passed-in consent data and will not operate if the `gdprApplies` flag is true.
 
+## Client-Side Token Generation (CSTG) mode
+
+For publishers seeking a purely client-side integration without the complexities of server-side involvement, the CSTG mode is highly recommended. This mode necessitates the provision of a public key, subscription ID, and directly identifying information (DII). In the CSTG mode, the module takes on the responsibility of encrypting the DII, generating the UID2 token, and handling token refreshes when necessary.
+
+To configure the module to use this mode, you must:
+1. Set `parmas.serverPublicKey`  and `params.subscription` 
+2. Provide **ONLY ONE DII** by set `params.email`/`params.phone`/`params.emailHash`/`params.phoneHash`
+
+### CSTG example
+
+Configuration:
+```
+pbjs.setConfig({
+    userSync: {
+        userIds: [{
+            name: 'uid2',
+            params: {
+               serverPublicKey: '...server public key...',
+               subscriptionId: '...subcription id...',
+               email: '[email protected]',
+               //phone: '+0000000',
+               //emailHash: '...email hash...',
+               //phoneHash: '...phone hash ...'
+            }
+        }]
+    }
+});
+```
+
 ## Client Refresh mode
 
 This is the recommended mode for most scenarios. In this mode, the full response body from the UID2 Token Generate or Token Refresh endpoint must be provided to the module. As long as the refresh token remains valid, the module will refresh the advertising token as needed.