When you use Back Stage's online submission system to apply to a casting notice, you have the option of including a cover letter with your submission package. Including a cover letter is a good way to engage the interest of the casting director, express your enthusiasm for the project, and include extra information about yourself and why you'd be a great person to cast.
To access your cover letters, visit your "Manage Your Resumes" page.
On your "Manage My Cover Letters" page you'll be able to see and edit all of your cover letters and create new cover letters. Click on the View/Eye button next to any of your cover letters to view each cover letter in full – the way your cover letter looks in that window is very close to how it will look to casting directors as well.