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+# Signals for Candidate Sources
+
+## Overview
+
+The candidate sourcing stage within the Twitter Recommendation algorithm serves to significantly narrow down the item size from approximately 1 billion to just a few thousand. This process utilizes Twitter user behavior as the primary input for the algorithm. This document comprehensively enumerates all the signals during the candidate sourcing phase.
+
+| Signals               |  Description                                                          |
+| :-------------------- | :-------------------------------------------------------------------- |
+| Author Follow         | The accounts which user explicit follows.                             |
+| Author Unfollow       | The accounts which user recently unfollows.                           |
+| Author Mute           | The accounts which user have muted.                                   |
+| Author Block          | The accounts which user have blocked                                  |
+| Tweet Favorite        | The tweets which user clicked the like botton.                        | 
+| Tweet Unfavorite      | The tweets which user clicked the unlike botton.                      |       
+| Retweet               | The tweets which user retweeted                                       |
+| Quote Tweet           | The tweets which user retweeted with comments.                        |
+| Tweet Reply           | The tweets which user replied.                                        |
+| Tweet Share           | The tweets which user clicked the share botton.                       |
+| Tweet Bookmark        | The tweets which user clicked the bookmark botton.                    |
+| Tweet Click           | The tweets which user clicked and viewed the tweet detail page.       |
+| Tweet Video Watch     | The video tweets which user watched certain seconds or percentage.    |
+| Tweet Don't like      | The tweets which user clicked "Not interested in this tweet" botton.  |
+| Tweet Report          | The tweets which user clicked "Report Tweet" botton.                  |
+| Notification Open     | The push notification tweets which user opened.                       |
+| Ntab click            | The tweets which user click on the Notifications page.                |               
+| User AddressBook      | The author accounts identifiers of the user's addressbook.            | 
+
+## Usage Details
+
+Twitter uses these user signals as training labels and/or ML features in the each candidate sourcing algorithms. The following tables shows how they are used in the each components.
+
+| Signals               | USS                | SimClusters        |  TwHin             |   UTEG             | FRS                |  Light Ranking     |
+| :-------------------- | :----------------- | :----------------- | :----------------- | :----------------- | :----------------- | :----------------- | 
+| Author Follow         | Features           | Features / Labels  | Features / Labels  | Features           | Features / Labels  | N/A                |
+| Author Unfollow       | Features           | N/A                | N/A                | N/A                | N/A                | N/A                |
+| Author Mute           | Features           | N/A                | N/A                | N/A                | Features           | N/A                |
+| Author Block          | Features           | N/A                | N/A                | N/A                | Features           | N/A                |
+| Tweet Favorite        | Features           | Features           | Features / Labels  | Features           | Features / Labels  | Features / Labels  |
+| Tweet Unfavorite      | Features           | Features           | N/A                | N/A                | N/A                | N/A                |       
+| Retweet               | Features           | N/A                | Features / Labels  | Features           | Features / Labels  | Features / Labels  |
+| Quote Tweet           | Features           | N/A                | Features / Labels  | Features           | Features / Labels  | Features / Labels  |
+| Tweet Reply           | Features           | N/A                | Features           | Features           | Features / Labels  | Features           |
+| Tweet Share           | Features           | N/A                | N/A                | N/A                | Features           | N/A                |
+| Tweet Bookmark        | Features           | N/A                | N/A                | N/A                | N/A                | N/A                |
+| Tweet Click           | Features           | N/A                | N/A                | N/A                | Features           | Labels             |
+| Tweet Video Watch     | Features           | Features           | N/A                | N/A                | N/A                | Labels             |
+| Tweet Don't like      | Features           | N/A                | N/A                | N/A                | N/A                | N/A                |
+| Tweet Report          | Features           | N/A                | N/A                | N/A                | N/A                | N/A                |
+| Notification Open     | Features           | Features           | Features           | N/A                | Features           | N/A                |                       
+| Ntab click            | Features           | Features           | Features           | N/A                | Features           | N/A                |
+| User AddressBook      | N/A                | N/A                | N/A                | N/A                | Features           | N/A                |
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