SCI fellowships are for spinal cord injury: care from the trauma bay to outpatient. SCI is a subspecialty of PM&R with it's own board exam. Only PM&R offers an SCI fellowship.
Spine fellowships are a collection of private and academic programs that focus on outpatient spine care, most teach injections in addition to comprehensive aspects of non-surgical care. These fellowships allow you to expand your practice and build skills but are not linked to any particular subspecialty exam. They can be offered by several specialty fields.
There are also a few "spine and sports" fellowships which usually allow you to sit for the Sports Medicine boards. Also offered by several fields.
There are also Pain fellowships which have a lot in common with spine fellowships, but a wider focus obviously on all types of pain. If the pain fellowship is ACGME cetified you can sit for the pain boards. Most are Anesthesia, a few PM&R but open in general to PM&R, Neuro, Psych, and Anesthesia.
Those of you in fellowship can correct me if I'm wrong on some of these details.