Abbreviating principle

I'm a bit confused about when the abbreviating principle is applied in Anniversary.
The 'small but useful group of words' in Unit 25: do they have anything in common apart from being common? Do you just have to remember them?
Unit 26: I can understand writing as far as the accented syllable, or beyond, if that makes it distinctive. 'abs' for absent makes sense. And if I check my dictionary, there it is: 'abs'. Same with essential, capable, frequent and so on. They're all there. But again, have they got something in common? If I pick other words at random from the dictionary (heliotrope, headboard, milkman, dynamic) the abbreviating principle hasn't been applied.
It all seems a bit random, which makes it difficult to get my head round. Have I missed something?

(by kevinwal for everyone)

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