Section A is a separate section of HBCA records, which contains the Governor and Committee (London Office) Records Finding Aid.
These records were created by the Governor and Committee of the Hudson's Bay Company, in London.
A listing of these records have, until recently, not been available to an online search -- I know that the British Columbia archives had a listing of the Section A records on microfilm.
They are now available online at the Hudson's Bay Company archives.
There is information here that is not contained in other records, and most if not all is available on microfilm.
However, you search for these records through Keystone -- that is, the Keystone Archives Descriptive Database.
Here is how the Hudson's Bay Company archives records were originally set up, in 1920 or thereabouts:
Section A -- London Office Records (Governor and Committee)
Section B -- Post Records, which contain surviving post journals and account books, and sometimes include Lists of Men employed at the HBC posts and Miscellaneous records. The catalogue listing is viewable online and for the most part can be requested on microfilm through your local library.
Section C -- Ships' Records -- the records of various ships that sailed up and down the coast, easily accessible by microfilm.
Section D -- Governor's Papers and Commissioner's Office Documents, including Governor Simpson's outgoing and incoming correspondence.
Section E -- Private records. For example, the Anderson Family papers are those of Alexander Caulfield Anderson's older brother, James Anderson A.
Section F -- Subsidiary Companies. The records of the North West Company are in this section, as are those of other companies attached, however tenuously, to the HBC.
Section G -- Maps, which can be searched online and viewed on microfilm. If you want to order a good copy of any map, please find the HBC Archives' instructions and read them carefully.
Section H -- Western Department Land Records, which I have not viewed but which are easily accessible.
Section Z -- Miscellaneous records, which are sometimes worth requesting to see what's available, though I think they are not always microfilmed.
Except for Section A, all of the above can be searched in the Hudson's Bay Company archives and can be requested, if microfilmed.
Only Section A is searched through the Keystone Archives Descriptive Database.
You will probably have to fool around with the database to discover how it works -- I know that I found some interesting material on there.
Amongst the HBCA's Information Sheets, I found this listing of records available in Section A:
Servants' contracts, 1780-ca.1926 -- A.32/1-60
Officers' and servants ledgers and account books, 1719-1922 -- A.16/1-114
Officers' and servants' wills, 1763-1921 -- A.36/1-15
Lists of HBC servants, 1774-1841 -- A.30/1-17
Servants' character and staff records, 1822-1832, 1851-1905 -- A.34/1-5, 8
By searching through the Keystone Archives Database, you will discover which reels to request from HBCA.
I suggest searching both for headings such as "servants' contracts" and also for your ancestor's name.
If I remember, the Keystone Archives Database is part of the Manitoba Archives page rather than the HBCA search page, which might confuse you for a moment or two.
But it works, and if there is a London record you should find it on this search page.