The SBL Accelerator courses run outside of and at additional cost to the SBL subscription - you get a small discount on the accelerator course if you do also subscribe to SBL. Enrollment is generally open for the accelerator courses every 12 months or so depending on the course.
It is a little frsutrating that the accelerator courses are at additional cost and going forward I'd consider dropping the SBL subscription and just choosing a standalone accelerator course when it is available. The accelerator courses tend to be a cut above the subscription courses in terms of organisation and structure although you could probably dig out the same information from across a number of the SBL subscription courses and You Tube videos.
Of course, "Accelerator" is really just SBL marketing jargon. All they really provide is the structure to allow someone to progress in a more structured and organised way and hence potentially progress more quickly .... if they put the practice in. Are they worth it? Each individuals mileage will vary but they do come with a 30 day money back guarantee to allow you to at least test the water (not that you cover much in the first 30 days). I've found them useful in terms of organising my practice which might say more about me than the course ...