If you are applying through direct entry you can only apply to one specialism, which you will need to indicate on the application form. If you are an in service applicant you can only apply for the nominated post from your employer.
Yes; you are still required to complete the application and demonstrate that you have the support of your employer, and the skills and experience required.
Direct entry are new posts created specifically for the purpose of training and open to competition by application. Applicants follow the whole programme full-time (except for any exemptions) during which they hold a fixed-term five year training contract of employment with the host organisation.
In-service are existing clinical scientist posts that are used by employers to support nominated individuals for HSST (as long as they reach the entry standard for HSST). They must hold a substantive contract of employment in the NHS specialism for which they are applying.
For direct entry applicants the interview stage is competitive however, for in service applicants the interview stage is for benchmarking and appointment.
You can check the progress of your application via Oriel, the online application portal.