Crenulate is apparently the diminutive of crenelate, so you should use crenulated for small things and crenelated for large things. The distinction between large and small is somewhat relative, but for instance leaves are crenulated, battlements are crenelated.
I have generally heard "crenulated" for living things and "crenelated" for inanimate things, but that may be because architectural features are large, and leaf and wing margins are small.
I too have searched for a definitive answer to this question and not found one. My own way of differentiating grammar from usage errors for my English language learners is as follows:
- If the mistake contravenes a generalizable rule for all members of that word class, then it is a grammar mistake. Otherwise it is a usage mistake.
For example:
He live in Frankfurt contravenes the rule that verbs in the 3rd person singular present simple tense require an -s (with the exception of modals), and is hence a grammar mistake.
My grandfather is a very high man is a usage mistake. We can formulate a rule that high applies to mountains not people. But the rule applies to one member of the word class only and hence the mistake is one of usage.
On this basis, these errors are grammar errors:
- I play tennis yesterday.
- Do you have dog?
- I live in Frankfurt since 10 year. (3 errors)
And these are usage errors:
- I always enjoy to sleep late on Sundays.
- What is the reason of your lateness?
- She replied she didn't know the answer.
The issue is of more than purely theoretical importance because learners need to know whether they should consult a grammar book or a good dictionary/usage manual to find out if what they have written is correct.
It is interesting to note that two excellent resources for English language learners both have the word usage in their titles:
Garner's Modern American Usage and Swan's Practical English Usage.
Garner's book exclusively contains what I personally would define as usage issues, while Swan's includes numerous entries on what I would term grammar; including negation, passive, modals, determiners, etc. So it seems that even the experts can't agree on the meaning of the word.
Best Answer
"special impact" implies that there is normally an impact of some sort, and so "special" means "some impact in this instance that is different to the normal impact".
"specific impact" would, I think, mean "clearly defined" in this context, and is actually (in my opinion) an example of a meaningless word which people insert into text to appear more clever, or to give some degree of plausible deniability if they turn out to be wrong, later, ie to "hedge their bets". This sort of language is very popular in a scientific or academic context, where people are often terrified of writing something which turns out to be provably false later.