The traditional career ladder of PRHO to SHO to SpR to Consultant is taken by the majority of doctors. However, a few fall by the wayside for various reasons and don't quite make the grade. The cuddly NHS, not wanting to lose these people, therefore from time to time creates new long term contract career grade posts for such doctors who are stuck on the career ladder, can't progress further up, and would have the next generation of doctors snapping at their heels if they were obliged to keep applying for the typical short-term 6 month junior doc post.
The BMA tends towards suspicion of these posts, noting for example that many Associate Specialists end up with full consultant responsibilities but for less pay and therefore worrying that the NHS is abusing these posts as a means to get cheap labour. Whilst they accept the argument that a place is needed for those doctors who fall off the career ladder, they keep a weather eye on the number of such posts created and filled. After all, there can't be that many stuck doctors and, if there are, maybe it reflects problems in the NHS's own training arrangements and support.
Associate Specialist is therefore the official title for the post occupied by somebody who couldn't quite make it to consultant, whilst Staff Grade used to be the term for somebody who couldn't make it to SpR.