add second column next to line number column

I would like to create a second column next to the line number column. It shall behave identical to the line number column, i.e., move up and down when scrolling. Ideally its content should be customizable using vimscript during runtime, because I want to display custom information in there. Where would I need to start looking? How is this column called? Is this possible?

Edit: Customizing how the line numbers are displayed, i.e. cheating in a second column by extending the first is also fine. Though I have not found a command for that.