Manipulating Outputs

Questions
1

I have written some code using the For instruction. I get a sequence of sets of positive integers as outputs. I want to create a set which is the union of those output sets. How?

2

It would be best if you included your code so we can see details.

3

Maybe

SeedRandom[314];
lists = RandomInteger[10, {5, 4}]
(* {{5, 7, 3, 9}, {6, 8, 5, 10}, {2, 0, 2, 5}, {6, 0, 10, 5}, {1, 8, 0, 
  8}} *)

Flatten[lists]
(* {5, 7, 3, 9, 6, 8, 5, 10, 2, 0, 2, 5, 6, 0, 10, 5, 1, 8, 0, 8} *)

Union[Flatten[lists]]
(* {0, 1, 2, 3, 5, 6, 7, 8, 9, 10} *)