IR tags explained ?

I have always be able to force my way through the tags in HL7, sooner or latter I could always get them to work in the translation.

Now I’m doing some X12 stuff.  

I pull up the xlate select the field I want to copy and run the tester.

It fails. I have tried every combination to see why it can’t match up the tags to what is says they should be. I could understand a blank field. But to give me a tag mismatch when I have don nothing but say select it through the gui baffles me.  

Does any one know of a good place where the IR tags and how they work are explained in detail ?