Thanks Dejan. When I asked this question, data security plans did not exist. It seems that "background backup" was accidentally added and set by default in 8.1, but it has reverted back to normal behaviour in versions since then.
Hello!
You can mark it as closed as option. If document is connected with other documents you can not delete it so you will not be able to delete PO, as you will not be able to remove Goods receipt...