Word order

[Sonyx]

I would like to know about the order of the word `necessary´ in the sentence.
Which is correct? or are both options correct? in

a. The documents necessary to travel, or
b. The necessary documents to travel.

Which is the grammar rule to place the word `necessary´before or after the noun in the case that both options are correct? is there any difference in meaning?

Thank you.

[Temico]

a. The documents (which are) necessary for travel......e.g. will be ready in a week.
b. The necessary documents to travel ......e.g. across the continent need to be processed in advance of the journey.

[Sonyx]

Thank you Temico for your explanation.
I understand, according to your examples, that if `necessary´goes after the noun, the phrase functions similarly to a defining relative clause, and if it goes before the noun it functions as an adjective, is that right?
Another question: shouldn't it be `for travelling´ instead of `for travel´in the sentence you used as the first example, as `for´is a preposition and hence followed by an -ing form?
Thanks.

[Temico]

shouldn't it be `for travelling´ instead of `for travel´in the sentence

You can use either,"travel" or "travelling". From my experience, native speakers use "travel" more often than "travelling".