Before I give you answers to your questions, I would like to teach you two things-they will be valuable if you really want to work in the US Healthcare system.
1. Do not ever use "text speak" when posting on a professional forum. It is unprofessional, and shows you lack communication skills-not an impression you want to create as an IMG.
2. Do not use an unprofessional sounding email address on a professional forum. Create a new one, maybe create bella84 @gmail.com, or whatever, but please do not use the one you have posted on your post-not for keeping in touch with ECFMG, not for anything other than private, personal emails with friends and/or family.
Now to your questions, "credentials" means "attestation of qualifications". Though you will send 2 photocopies of your original medical college certificate (ECFMG language-Medical School Diploma) that states that "bella84 passed her MBBS from x medical college affiliated with y university in year z", ECFMG still wants to make sure that you were issued those "credentials" (MBBS in your case) appropriately. Hence it verifies from your school as well. The only "credentials" ECFMG accepts from India is MBBS, not BDS/BAMS/Homeopathic degree, whatever else, only MBBS-that is what they are looking for on your "diploma".
Form345 is the Medical School Release Request that you will send to ECFMG that they will then send to your school which will fill out your performance details based on your marksheets. You do not have to send your marksheets or their copies to ECFMG.
Transferred credits almost always don't apply to Indian medical students. It means if you did part of your MBBS-say 1st year in one college in one city, then moved to a different city/college and started from 2nd year, then you have to send information about both colleges you attended. I am pretty sure you won't have to worry about that.
Clinical clerkships are the 3rd, 4th and 5th year rotations you do in the various departments. I am not sure how essential they are, I didn't put mine in (I did mine a long time ago, don't remember the exact details), and my paperwork still got processed. However if you are fresh out of medical school, you might as well enter everything, it will be a good record for your own future as well.
The most important form to get done correctly is the Form 186-read up instructions for it very carefully, and make sure everyone does their part correctly. Usually if you take the form to the Dean's office, they will be aware of the appropriate person/procedure for it as long as you are not the first person from your medical college to apply to ECFMG for USMLE. Once completed, it should be posted by your medical college to ECFMG. One way to ensure it happens in a proper and timely manner is to get it signed in your presence, then send it via courier to ECFMG with your Dean's office as the sender (sender's address-Dean's office...), and you can pay for that expense. If however, you are far away from your medical college, and a Notary Public signs your Form 186, you can send it with the other forms and documents to the ECFMG yourself.
Hope all of this helps. And please, heed my advice about points one and two above. They are essential for your professional career.
