The VMS documentation is a joke I am afraid, you will have to download the PDF of Vegas Pro where this issue is explained there in a VISUAL way, which is better than me trying to explain it to you via text.
Interesting. There's always something to learn. It's now on page 60 of 430, or 56 Ch 2.
I've also wondered how to preserve the sequence (alpha or date) when a group of clips/pics are dragged to the timeline ?