Frequency: The word is one of the 3000 most common words in English.
1.
If you say that something is not necessarily the case, you mean that it may not be the case or is not always the case.
Anger is not necessarily the most useful or acceptable reaction to such events...
A higher fee does not necessarily mean a better course.
ADV: with neg, ADV group, ADV before v [vagueness]
•
If you reply 'Not necessarily', you mean that what has just been said or suggested may not be true.
'He was lying, of course.'-'Not necessarily.'
CONVENTION
2.
If you say that something necessarily happens or is the case, you mean that it has to happen or be the case and cannot be any different.
The most desirable properties necessarily command astonishingly high prices...
Tourism is an industry that has a necessarily close connection with governments.
= inevitably
ADV: ADV before v, ADV group