adding description for 'Use Existing Pixel' setting

Author: joe-ayoub-segment

src/connections/destinations/catalog/actions-tiktok-pixel/index.md

@@ -26,11 +26,20 @@ This destination is maintained by TikTok. For any issues with the destination, [
 4. Select an existing JavaScript Source to connect to TikTok Pixel.
 5. Give the Destination a name.
 6. On the Settings screen, provide the Pixel Code. This can be found in the TikTok Events Manager (TTEM).
-    - Ensure that the Use Existing Pixel field is **off** if you want Segment to load the TikTok Pixel JavaScript code onto your website.
-    - Ensure that the Use Existing Pixel field is **on** if you want Segment to detect and keep using the existing TikTok Pixel JavaScript snippet which has already been loaded on your website.
+    - Ensure that the "Use Existing Pixel" setting is **off** if you want Segment to load the TikTok Pixel JavaScript code onto your website. This is the default and recommended behavior. 
+    - Ensure that the "Use Existing Pixel" setting is **on** if you want Segment to detect a pre-existing TikTok Pixel which is already loaded to your website independently of Segment. If you enable this option and do not have TikTok Pixel loaded to your website, your data collection to Segment may be disrupted. See [The "Use Existing Pixel" setting](#use-existing-pixel-setting) for more information on how to use this setting safely.  
 7. Toggle on the Destination using the **Enable Destination** toggle.
 8. Click **Save Change**.
 
+<a id="use-existing-pixel-setting"></a>
+## The "Use Existing Pixel" setting
+
+The TikTok Pixel Destination's default behavior is to load a TikTok Pixel Javascript library onto your website when the page loads. However if you already have TikTok Pixel on your website before enabling this Destination, you might prefer that Segment detects and uses your pre-existing TikTok Pixel Javascript library instead of loading a new TikTok Pixel library. 
+
+Toggling "off" the "Use Existing Pixel" will prevent Segment from loading a TikTok Pixel library onto the web page. Instead, Segment will attempt to detect your pre-loaded TikTok Pixel library and will use it when forwarding events to TikTok. 
+
+Care should be taken when toggling the "Use Existing Pixel" setting to "off" as doing so could result in disruption to data collection if Segment doesn't detect TikTok Pixel on the website. 
+
 ### Mappings enabled by default
 
 After setting up the Destination, Segment enables seven mappings by default. You can click on the mappings tab to view and edit these mappings.