That s right Terminal and non terminal functions only handles CFL without saying so as many wikipedia articles do It is mainly about parse trees and uses some unusual notation viz function which occurs neither nor the only source nor in the parse tree article Maybe a sentence that nonterminal symbols and terminal symbols correspond to interior and leaf nodes in a CFG parse tree respectively could be used And the example from the picture seems more understandable everyone who learned English grammar might have seen such a tree than the example which doesn t fit the previous grammar definition but uses some unexplained extensions for optionality and repetition It could look like this wording subject to improvements Nonterminal symbols and terminal symbols correspond to interior and leaf nodes respectively As an example consider the following context free grammar for simple English sentences N S NP VP Det N V John ball hit the start symbol is S P is as follows S NP VP VP V NP NP Det N John N ball V hit Det the In this example nonterminal symbols denote grammatical categories like sentence abbreviated by S noun phrase NP etc while terminal symbols are English words like John ball etc