If there are unresolved issues in a chat, you can use the “pin” feature to create a visual reminder. Pinned chats always stay on top of the chat list and are marked with a pin icon.
Android: In the chat overview, swipe from left to right on a chat until the pin appears on a yellow background. Release to pin or unpin the chat
iOS: In the chat overview, swipe from left to right on a chat and tap the pin icon to (un)pin the chat
Desktop app / web client: Open the desired chat, and tap the pin icon in the top right corner to (un)pin it