Variables and Arrays from main to subreport

First of all, forgive me for my cumbersome way of explaining this.

I have report that shows monthly usage of inventory and a subreport that shows when each item was ordered. The main report is grouped by "sites", which is a formula field (@site). The subreport is grouped by location, (@POInvLoc) which is ultimately a different formula that creates the same "sites", except there is an account number associated with the site as well.

I have the reports linked together fine, except that an item could have been ordered in multiple sites. I want the subreport to only show orders for the site where the item was used. Do I need to create an array variable or just a variable? I'm having trouble creating either.

And can you pass a variable (or array) from a main report to a subreport? How do I do that?

Help!!

One more unrelated question.....I have a max date summary on the subreport to show the last date an item was ordered (for each site). If I want to show say the last 3 times an item was ordered, can I edit the formula to do so?

Thanks, Cindy