Let there be given this sentence (which came from an English-Chinese dictionary):
The contest has become personalised, if not bitter.
Then what does the phrase if not mean?
Seeking after is a general guide or rule of such usage.
Let's look at simpler example -
This means if all chapters are not possible, try to finish at least 10.
This means that that smell is likely to cause vomiting but if it does not, at least it can causes nausea. In other words, that smell is capable at least to cause nausea but it can also go closer to vomiting or in worst cases, it can cause vomiting.
In such cases, the part B is expected or desired but then actually part A is likely to happen.
This simply means the latest will be 1900 hr. The speaker wants to say that he'll try to pick the listener earlier but not later than 7 pm.
Another such example may be - I'm a good tennis player, if not a great one.
So, in your sentence, the contest did not turn bitter but at least got personalized.
A later edit (from J.R. and user42307's input): The sentence may also mean that the contest is on the verge of getting bitter (nausea's example) or has become bitter.
Unfortunately the phrase is used in two ways, and especially when written it can be difficult to distinguish them. The hypothetical "if" might mean that "bitter" is acknowledged not to be true, or it might just mean that there's doubt.
So the first meaning is that the contest has not become bitter, although perhaps is close to it. The phrase creates a contrast, and could be replaced with "the contest has become personalised, but not bitter" or "personalised, but not quite bitter".
The second meaning is that the contest probably has become bitter, but that this opinion isn't certain or might be disputed. Then the phrase is used to make a certain statement followed by the less certain one. It could be written, "the contest has certainly become personalised, and quite possibly bitter".
When spoken, listen for which word is more stressed:
"The contest has become personalised, if not bitter" means it's not bitter (or probably not).
"The contest has become personalized, if not bitter" means it's probably bitter (or certainly so in the speaker's opinion, but they choose not to assert it).
"The contest has become personalised, if not bitter"
-The contest has become bitter.
"As a worker, he's lazy and arrogant, if not downright dishonest"
He's downright dishonest.
"I'm disappointed, if not shocked, at your behaviour."
-I'm shocked at your behaviour.
The wide and varied English language gives us this "Get out of jail free" card. Isn't it wonderful? Enjoy!
[Edit] Correcting my mistake in thinking "if not" is always pejorative...
The following, Nothing if not variation might also help:
"She's nothing if not generous"
-She's generous -And I won't tolerate any contradiction of that.
Then there's the "a little" modifier:(without the "not)
"He's diligent, if a little slow."
-He pays so much attention to detail it slows down his work
(or with the "not"):
"He's diligent if not a little undervalued."
-He pays attention to his work and his talents are being wasted on this menial work
Or the use of both the "at least" and the "if not a little" modifiers:
"At least the band is relentless if not just a little funny."
-The band isn't very good but never gives up and makes me laugh.
The sentence containing "if not" is a short way of expressing the following sentiment:
[A] if not [B].
[A] is certain while [B] is not certain but may be possible. The contest has certainly become personalized. It might also be getting bitter.
Usually [B] is a more extreme interpretation of A.
My answer is good, if not the best.
If not usually is placed in a sentence to compare the sentence value with another.
Take the following.
A: The food in my house is good B: the best
The food in my house is good, if not the best.
The comparison of my food in my house is being done with the value of being "the best". Therefore the statement says that my food is good, if not the best. This lightly refers to my food being close to the best value through the "if not". So the use of "if not" allows one to compare values with an adjective whilst using another adjective.
Adjective A: Good Adjective B: Best
I am stating that my food is describable by both adjectives but I put adjective B at a higher value by saying "If not". It may seem a bit confusing but it allows for the use of multiple describing objectives to a subject without the need for a break in the flow of the sentence.
Hope the answer helps.