Where can I find technical information about the FFEplus Solver?

I'm interested in learning more about the FFEplus solver, specifically it's convergence methods (not in adaptive mode). Are there resources available to us that I can read more about the technical aspects of FFEplus? So far I have found minimal information on the knowledge base. I know there is a technical manual for FE equations available, but I'm looking specifically about how the solver measures error, iterates, etc. 

 

Thanks in advance!

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