How do you create ignore sub & Exclude in Contextual Chaining Substitutions in Fontforge?

I am trying to understand when to write ignore sub and the problem I have is that I don't know where exactly I can create them in FF. Can anyone explain why we need it and how and where I can create one in FF. I sort of understand there are exception cases in the Calt feature, where I need to write for it, right?
And how to create Exclude sub languages in FF? I can write it in text file and merge it to FF, but is there any other way thru GUI? Thanks 

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