All available data on an order is usually determined by the channel (not taking test orders in account) - it will contain all the information like the customer information, the ordered products and order totals. However, in some cases, you might want to add some sort of note on an order. For example, if it needs a manual check later, you changed the email address manually or had ChannelEngine manually change the status of the order.
For these situations, you can use the (merchant) order comment that you can create, edit and remove through both the ChannelEngine web interface and API.
In the overview, the comment option is visible in the 'Actions' tab (location on the right-hand side) and marked by the talk cloud icon. If the icon is solid (dark), this means a comment has been added (hovering over it will show the current comment). If the icon is outlined (so it is mainly white) it will not yet have any comment. By selecting the icon (regardless of the color), a popup modal will become visible in which you can edit the comment.
In the order view for a specific order, you will see the 'order comment' in a separate section below the personal details section on the top of the page. By clicking the pencil, a similar popup modal will appear as in the order overview and you can add or edit the comment (press '+ Update' to save).
If you want to clear the existing comment, just make it empty and save.
In any GET /v2/orders call, the MerchantComment will be included. If you want to update or add a user comment, the PUT /v2/orders/comment can be used. Please note that this call requires at least one order id reference to be included, which can be either the ChannelEngine order id or your own merchant order number (but this will not exist for an order still marked as 'New').