I was having a problem trying to put a 100% white fill in a rectangle to create a mask over some lines within the rectangle. No matter what I tried, the lines I was trying to hide ( that were on a different sheet to the rectangle and fill ) wouldn't be obscured.
Problem was solved when I was informed that the position that a Sheet is located in the Sheets list, affects it's level on the drawing. When I moved the sheet with the rectangle and fill, higher up the list than the sheet with the lines that I wanted to obscure, I had no further trouble with what I was trying to do.
Hope this is as enlightening for some others as it was for me.