Background. As I assume that you have very little knowledge about the military in general, can I make the point that the Royal Marines is a separate service which started life as marine infantry. They have very little to do with the Army and would be most offended to be referred to as soldiers. They have their own distinct history and customs, and even their ranks, which although they have the same names as some Army ranks, are not always direct equivalents. To get some more specific background on Thomas Attwood’s service, can I suggest you read this
Wikipedia article which describes the genesis and development of the Commandos. They were initially formed from across the Army and Royal Marines and it wasn't until later in the war (February 1942) that the Royal Marines changed their structure to form a number of Commando Brigades (Cdo Bde), each of which were made up of a number of Commando units. A Commando is a unit roughly equivalent, although smaller, to an Army Infantry Battalion.
By the time Thomas joined, this reorganisation had already occurred but we still see a remnant of the old RM system of organisation in his number CH X 11815. Here the CH stands for Chatham Division. The RM were previously split into 3 Divisions: Chatham, Portsmouth and Plymouth. A Marine would then have spent the whole of his career in the same Division. This changed with the creation of the Commandos, and as can be seen from Thomas’s record, a marine might be moved between Commandos to meet specific operational needs. However throughout his service he appears to have remained within 4 Commando Brigade. This formation was mainly tasked with operations in and around North West Europe.
Service Record. His record only tells you which unit he was with at any one time. It tells you nothing much about how or where he was employed. For that you could start by looking at the Wikipedia entries for the various Commandos mentioned in his record, mainly 46, 41 and 47 Cdo. As you can see from the first article I linked to, the commandos or elements from them were used to carry out small coastal raids (such as the Dieppe raid in 1943) and also in much larger groups to secure bridgeheads and landing beaches, like the Normandy landings. However, as you have seen, the period 8 May 1944 to 21 Jun 1944 is blanked out on his record sheet. so we have no idea what he was doing in this six weeks, which obviously cover the D Day landings. We do know that he returned from France on 27 June, after his Commando had suffered heavy casualties during the breakout from the Normandy beaches.
Abbreviations. Rather than try to decode his record line by line, I will explain as many of the abbreviations as I can and you should be able to at least recreate the sequence of events. Bear in mind that my background is in the Army and although most of the administrative procedures and terminology were the same, it is possible that the details may vary in the Royal Marines.
SS Bde – Special Service brigade. This was the early title for what were to become Commandos.
Y list and X list. The Y list is an administrative record for holding a serviceman who is not available for deployment, perhaps due to being untrained, long term sickness or being missing or a prisoner of war. The X list performs a similar function for soldiers who are available to be deployed but are not currently on the strength of a fighting unit. In particular X (iv) is a category for a man who is in a holdiong unit awaiting transfer to a new unit/employment. As can be seen from Thomas's record the use of holding units was very common in the Royal Marines, due to the fact they were constantly reorganising to meet the different operational requirements.
ITC – Initial Training Centre. This was (and still is) based at Lympstone near Plymouth. Also home to the Depot Royal Marines where he spent a brief time.
HBL Not sure what this stands for It was obviously some sort of holding unit, possibly a forerunner of the HOC.
HOC - Its formal title was Operational Holding Commando HQ
CHU – Possibly Chatham Holding unit
Emb NWE – Embarked for North West Europe.
Disemb UK - Disembarked in the UK
1st GCB - First Good Conduct Badge. These were awarded after the first two years of satisfactory conduct without any significant disciplinary occurrences. Had he stayed in he might have expected to get his next GCB after 6 years service. The award earned him an extra penny a day on top of his basic pay.
WSI - War Service Increment. This was an additional payment based on rank and length of service during hostilities, in Thomas’s case this was for 3 years service at the rate of 1/- (1 shilling) per day. I am not sure what the A&S [?] reference means. The following line relates to his commando pay which I assume was 4 shillings and 9 pence per day.
Class A release [to the Reserve] - Class A was the standard release whereby servicemen and women were released in accordance with a formula based on age and length of service. In other words he would have been released to go back to civilian life after Marines who had joined up before him.
I hope that helps you. Please come back with a specific entries you don’t understand.