Unmarked money...?

Question

When they talk about "unmarked money" what exactly are they talking about? Are the bills lacking serial numbers or are they devoid of hidden markings?

Answer 1

They don't have hidden markings, like someone taking an ultra violet pen and just sticking a mark on them so they can be later identified.

Answer 2

Some times banks and others place markings which are at times invisible to the naked eye on money./ They do this to be able to identify the bills at a later date.

Unmarked money has none of these markings.