custom identifier section

Author: forstisabella

src/unified-profiles/segment-for-flex.md

@@ -106,7 +106,7 @@ Connect to either a website or mobile app to complete this step.
 5. After Segment marks the "Add connections" tile as complete, click **Add identifiers and traits** and begin [Step 3: Add identifiers and traits](#step-3-add-identifiers-and-traits).
 
 ## Step 3: Add identifiers and traits
-After you've selected which data sources you'd like to integrate customer data from, you can select _identifiers_, or unique pieces of data that allow you to link information about an individual customer across different programs and services, and _traits_, which are pieces of information you know about a particular customer.
+After you've selected which data sources you'd like to integrate customer data from, you can select _identifiers_, or unique pieces of data that allow you to link information about an individual customer across different programs and services, and _traits_, which are pieces of information you know about a particular customer. In this step, you can select one or more of Segment's 11 default identifiers. To add custom identifiers to your workspace, see the [Add custom identifiers](#add-custom-identifiers-to-your-workspace) documentation.
 
 1. On the Add identifiers and traits page, click **Add identifier**. 
 2. Select one or more of Segment's 11 default identifiers and click **Add identifiers**.
@@ -133,7 +133,7 @@ If the Enable Sources and Test Connections check indicates there are problems wi
 - [Profile Explorer](/docs/unify/#profile-explorer){:target="_blank"}: Use the Profile Explorer to view all user data, including event history, traits, and identifiers. 
 - [Advanced Segment](https://app.segment.com/goto-my-workspace/overview){:target="_blank"}: Use the Advanced Segment option to view your full Segment workspace. Segment recommends working with the assistance of Professional Services when accessing Advanced Segment.
 
-## (Optional) Add additional sources and destinations to your workspace
+## (Optional) Add additional sources, destinations, and custom identifiers to your workspace
 
 After you complete the Segment for Flex guided setup, you can use [Advanced Segment](https://app.segment.com/goto-my-workspace/overview){:target="_blank"} to connect your workspace to additional *sources*, or websites, server libraries, mobile SDKs, and cloud applications that can send data into Segment, and *destinations*, or apps and business tools that can receive forwarded data from Segment.
 
@@ -164,6 +164,16 @@ To add a destination to your Segment for Flex workspace:
 3. Select the destination you'd like to add to your workspace, and click **Next**.
 4. Follow the setup flow, and click **Done** to complete setting up your source. 
 
+### Add custom identifiers to your workspace
+
+You can add an unlimited number of custom identifiers to your workspace in Advanced Segment mode. 
+
+To add custom identifiers to your Segment for Flex workspace: 
+1. Open your Segment for Flex workspace in [Advanced Segment](https://app.segment.com/goto-my-workspace/home){:target="_blank"} mode. 
+2. Select **Unify** in the sidebar, click the Unify space you created during the guided setup, and select **Unify settings**. 
+3. On the Identity resolution page, click **+ Add identifier** and select **Custom identifiers**.
+4. On the **Custom Identifier** popup, walk through the steps to create your custom identifier. When you've finished setting up your custom identifier, click **Add new identifier**. 
+
 ## Segment for Flex entitlements and limitations
 
 Segment for Flex workspaces created during the Unified Profiles setup process have the following entitlements and limitations: