This option provides a list of the printers that you have defined to SAS/LAB software. When you select a printer from this list, it becomes the active printer. That is, when you select 'Print current text' or 'Print saved text' from a Journal pull-down menu, your journal text will be sent to this printer using the commands that you have provided.
A field at the top of the Select Active Printer window shows the current active printer. When you select a different item from the list, this value changes. You can use the button to send a small test file to the current active printer to see whether your printer commands for this printer are working.