1. The primary button helps to highlight the main action of the page.
2. The secondary button should be used for actions that are less important than the main action.
3. The ghost button (also known as text button) is a specific secondary action. It should be used when the action is recurring and linked to patterns or a group of elements (e.g.: a table, a card list, a visual, etc.)
Add an icon to the label when the words are not clear enough, help with understanding and give more meaning to the action. (Label + icon)
Use icons when their meaning is clear enough for the user. (Icon)
4. Feedback buttons are optional and allow you to give feedback to the user about the action. For example, to indicate that the action is dangerous (e.g.: delete a file).