When an employer specifically requires a cover letter and resume, that's what the job seeker should submit. It's relatively rare than an employer will accept an application letter in lieu of a cover letter and resume. The two approaches are different and employers use application letters infrequently, when compared to how they use cover letters and resumes. An application letter is more appropriate for unsolicited interest, or when there isn't a job posting. For example, an application letter could be mailed to several employers that aren't advertising specific jobs vacancies as a way to provide a more extensive introduction to prospective employers.