SETSELECTIONFILTER in Actions

So, It was asked to me as learning exercise, to create an action in a page to pick a boolean Field based on the selected lines, so for exemple:

3 lines selected = Boolean Field of each line picked.

I created the code of this action in a codeunit to select the lines and fill the boolean field, but only fills the boolean field of the last line, I have 3 selected and the last one gets the boolean field filled. The action code only have 2 lines

CurrPage.SETSELECTIONFILTER(Rec); - This was supposed to make the function below to work on more than 1 line selected.
CUTest.PickLines(No.); - No. is the primary key of the table as parameter to know which lines are selected

Help me understand how should I use SETSELECTIONFILTER in this case and if I have to leave the SETSELECTIONFILTER in the page or in the codeunit with all the code.

P.S - I believe there's no need of showing the CU code here, but If you need to see it just ask.