When referring to printers the in-tray is the area one loads blank paper and the out-tray is the area finished prints come out of and lay to dry.
When referring to scanners and fax machines, the in-tray is the area one loads a document to be scanned and the out-tray is the area the document is ejected from.
When referring to communications such as e-mail or post the in-tray is an area reserved for incoming messages usually addressed to oneself, and the out-tray is an area for outgoing messages .