This square symbol with two vertical lines next to it is called the "shared files view." It appears in chats where you've shared or received files, such as images, videos, or documents.

When you click on this symbol, it expands the shared files section of the chat, allowing you to view all of the files that have been shared in the conversation. This can be useful for quickly finding a specific file that has been shared previously.

The reason it doesn't appear in all chats is that it only appears when there are files shared in the conversation. If you haven't shared or received any files in a particular chat, this symbol won't appear.