AVC in general - http://www.longlongtrail.co.uk/army/regiments-and-corps/army-veterinary-corps-in-the-first-world-war/
DAC usually means Divisional Artillery Column to me
- warning - it may have other meanings that I don't know about. If
DAC really is Divisional Artillery Column then it may well be that he was attached
to the 21st Divisional Artillery Column from the AVC. And I did just find a reference in one of my little books that described the organisation of a Royal Field Artillery battery where it stated that the medical and veterinary
guys were attached from outside the RFA. So attachments are certainly possible for that sort of role.http://www.longlongtrail.co.uk/army/order-of-battle-of-divisions/21st-division/
refers to the 21st Division whose DAC was apparently called the 21st Divisional Ammunition Column RFA (I was prepared for a more challenging relationship with the numbers!) - and the 21st Division crossed to France in early September 1915 - coincidence? If this was his unit, then that URL gives you a basic history.
I didn't see anything in his Service Records to clarify whether my suggestions are true but it was a quick look only, so I might have missed something.
Now - a personal plea to people asking queries about Military - can you please
give us the full details of name, unit and number as you've found them in the records? I spent some time looking for William Orbell before I found that he was actually Willie Orbell in the Army. Had it been Jones, I'd have given up before finding him....