Choosing different printer trays

Hello,

I'm wondering if someone has a solution for choosing between different trays of a printer in this situation:

Tray #1:  8.5 x 11 white paper.

Tray #2:  8.5 x 11 transparency vellium.

The Illustrator reference doesn't list a way to choose between trays. The only way I can see to select the paper you want is by size.  But what if they are the same size, just different material? 

I'm wondering if I would have to set up a custom paper in the printer itself and have the printer report it in a "PaperInfo" call?  Or, is there something that isn't in the reference that would enable me to choose the tray I need? 

Thanks for any and all info!