The RDG document came up in a Google search and I know I've got the leaflet we all got when the licences were introduced somewhere but I don't seem to have an electronic copy to hand.
Further digging has turned up this document which refers to the subcategories in Annex 2
"1.3.Categories of driving
The categories and types of driving, for which the driver is authorised to drive, shall be displayed as follows.
Category A
RUs/IMs may use ‘A’ as a global category, encompassing all activities of category A: shunting locomotives, work trains, maintenance railway vehicles and all other locomotives when they are used for shunting.
Alternatively, RUs/IMs may limit the scope of the certificate to one or more of the following types:
A1=when limited to shunting locomotives;
A2=when limited to work trains;
A3=when limited to maintenance railway vehicles;
A4=when limited to all other locomotives when they are used for shunting;
A5=other, if the authorisation is related for services or rolling stock not included in the previous categories. This shall be specified in the appropriate space.
Category B
RUs/IMs may use ‘B’ as a global category, encompassing carriages of passengers and goods.
Alternatively, RUs/IMs may limit the scope of the certificate to one or more of the following types:
B1=when limited to carriage of passengers;
B2=when limited to carriage of goods.
The category in which the holder is authorised to drive shall be indicated by filling in the appropriate box if a global category is chosen or boxes, if one or more subcategories are chosen. The boxes that are not necessary shall be made void".