No SecureSource and SIM are not the same thing.
SecureSource is a Welss Fargo partner program. from the AIM glosery page:
An Authorize.Net partner program for merchants in need of a merchant bank account. This program includes additional integration requirements. For more information, see the Wells Fargo SecureSource Developer Guide at SecureSource_guide.htm
SIM is the Remote payment method for Authorize.net.
Under the Remotely Option, select the Credit Card option to use the SIM service.