The Data Access Layer needs to supply a User Name and Password in the connection string in order to connect to the data store. This User Name/Password can be hardcoded in the connectionParms.config, but normally a developer turns this off by removing the hard coded values and requires each service call to supply the logged in credentials. The Accelerator core automatically passes these credentials in many cases, but there are situations with custom components where you will be responsible for passing these parameters.  The logged in credentials are available in the AB_DataPropertyController class and can be accessed like this:

var result = SBInterfaceVm.am_Select(new AB_SelectInputArgs<AB_StandardBaseInterfaceEntity>(sbQuery, AB_DataPropertyController.ap_UserIdentityStore.ap_UserName, AB_DataPropertyController.ap_UserIdentityStore.ap_Password, AB_DataPropertyController.ap_Token));
if (result.ap_IsSuccess)

Note: It is recommended to pass the UserName, Password and Token.  The token is a variable that will be used in a future enhancement where the Accelerator Database Authentication will support tokenized security.