A green pin on the right side of the matter indicates whether the user is subscribed to that specific matter. When the pin is greyed out the user can subscribe to the matter by clicking the pin. The pin will now turn green and the matter will appear under My Matters.
By visiting My matters users will find slightly different pins on the right side of the matters. By clicking a pin the user will unsubscribe from the specific matter and the matter will disappear from My matters.