Full Idea
In addition to absolute necessity ('p is strictly implied by ¬p'), i.e. p strictly implied by any proposition whatever, C.I. Lewis also distinguished relative necessity ('p implied by what is given or known').
Gist of Idea
There is 'absolute' necessity (implied by all propositions) and 'relative' necessity (from what is given)
Source
Harré,R./Madden,E.H. (Causal Powers [1975], 7.V)
Book Reference
Harré,R/Madden,E.H.: 'Causal Powers: A Theory of Natural Necessity' [Blackwell 1975], p.132
A Reaction
Once you accept this distinction you find that the 'relative' one comes in all sorts of degrees. You "have to" put more salt in this soup. (Deontic' necessity, someone on Twitter tells me!)
Related Idea
Idea 19286 'Absolute necessity' is when there is no restriction on the things which necessitate p [Hale]