'a' is used when an input u8 is not 0x3, otherwise 's' is used. This is [[Content_Manager_services#Title_Types|title-type]]? A seperate function using the ".../c/" URL is hard-coded to use 'c'. This appears to match 's' usage attempts: the only URL that returned actual data with 's' was with titleID 0100000000000816.
Going by strings in NIM, it appears SOAP isn't used anymore. ecs appears to use REST API?