Thank you for that. I should have indicated that the previous example code is not mine, it's from the SciMark2 tutorial in the library.

After going back to the SDK for more information on what's in your post, I found that the numbers in the DoMethod() calls refer to how many more words are being passed as arguments. eg:

DoMethod(findobj(OBJ_BUTTON_LGD, App), MUIM_Notify, MUIA_Selected, MUIV_EveryTime,
App, 3, MUIM_Set, APPA_LargeData, MUIV_TriggerValue);

the '3' indicates the number of remaining parameters.