App messages can make use of some more advanced formatting options, allow your app to build richer interactions that a plain-text message.
Message attachments allow you to add richer formatting to messages, as well as interactive buttons and menus. Try to use the least amount of formatting needed, and remember that messages will look slightly different on mobile devices than they do on a computer screen.
Don't use an attachment when regular message text will suffice, and don't send multiple attachments when a single attachment will do.
And never ever (ever!) send more than 20 attachments.
The difference is small, yes, but Slack prefers consolidated messages.
Each attachment should represent an object so if there's a title for it, it should be inside the attachment.
Please don't display a long list of items. Try to show the most likely options first and, if you must, use buttons to paginate items. Try to replace the list instead of adding new messages.
Avoid cluttering up the conversation for everyone by using ephemeral messages in conversations when displaying items as an intermediary step of an action.