As it often happens with the use of articles, we need to make certain assumptions about whether both parties understand what is being referred to.
There is a phrase in a technical manual:
Select the necessary value from the dropdown list
On the one hand, the motivation for using the definite article is that the user must know which of the values is right so this value is rather definite and exclusive.
On the other, the same phrase with the indefinite article seems logical to me as well:
Select a necessary value from the dropdown list
We do not know the value yet so the user is asked to select some value on the list that they find necessary in this particular case.
Which of the options would you use in this case and why?