It's saying that he was classified as (B1, A3), again as the same status, and then just (A3) before moving out again (he was in the UK at the time).

Now you may be asking "what do those number and letter classifications mean?". They are in fact medical classifications.

There were a couple of major classifications: A, B, C, D, and E. A was "Able to march, see to shoot, hear well and stand active service conditions.".

B was "Free from serious organic diseases, able to stand service on Lines of Communication in France, or in garrisons in the tropics".

C was "Free from serious organic diseases, able to stand service in garrisons at home."

D was "Unfit but could be fit within 6 months.".

E was "Unfit and unlikely to become fit within 6 months."

Within each of these there were four subcategories, 1-4 respectively. With 1 being the best of the classification (for example, A1 is a man fit for active service) and 4 the worst.

B1 was "Able to march 5 miles, see to shoot with glasses, and hear well" and A3 was "Returned Expeditionary Force men, ready except for physical condition".

These classifications also would tell when a man would (or would not) return to fighting. B1A3 was "B1A3 Fit for overseas training camp in two to three weeks"

and then A3 meant "Fit for overseas training camp, to which transferred for hardening, prior to rejoining unit overseas" (so ASAP basically).

Your Great Great Uncle had gotten Trench Fever (went to Number 9 Casualty Clearing Station), then transferred to the 10th General Hospital, before being sent to England for recuperation. That process started on May 26th, 1917. He had been at the front for about 6 months at that point, as a part of the 2nd Pioneer Battalion (Australia). This means he was doing a lot of physical labor, digging, carrying, that sort of thing. He was in England from June until September 21st when he arrived back on the Continent and rejoined his unit.

If you would like to know more about Pioneer Battalions (and their often underappreciated role in the war) check out Pioneer Battalions in the Great War by K. W. Mitchinson. While he mentions only Pioneer Battalions from England, the work and conditions of those Battalions would be the same as ANZAC Pioneers.

Cheers!

EDIT: Figured I'd add on what Trench Fever is as well. At first medical authorities were baffled as to why men were coming down with an odd series of flus. They ranged from aches in the legs, to serious headaches and high fevers. Regimental Medical Officer Charles McKerrow studied the disease and the results of his effort was a paper called Pyrexias of Doubtful Origin in an Infantry Battalion on Active Service. While he offered no definitive cause in 1918, it is now commonly believed to be carried by Lice.

---

Sources

• Wounded by Emily Mayhew, pages 54 and 55.

• Soldiers Song and Slang of the Great War by Martin Pegler, page 189.

• As well as the paper linked above.

• I don't have any books that cover the medical classifications in detail, all the specifics of classification was retrieved from 1914-1918 Forum The posters in that thread have specifics as to where their info was retrived (For example, Australian Veteran's Affairs). I seem to remember them being mentioned in Tommy by Richard Holmes so I'll look through that over the next couple of days.

Could you give us more information? Country of service, Regiment or Btn, role, etc?