iEngine vs Engine Class

Hi All,

I wanted to programmatically compare 2 sequence files. I found a "DiffSequencecFiles" method in EngineClass. But I have a few questions, 

1. What is the difference between Engine Class and IEngine? The Engine Class has the GetSequenceFile method and the IEngine doesn't have the GetSequenceFile or DiffSequenceFile methods. 

2. In which case should the Engine Class and IEngine be used?

2. What is the difference between GetSequenceFile and GetSequenceFileEx? 

Thanks,

Sree Ranjani