Card hoppers (input) and stackers (output) are represented by a small, recessed, panels with a white background. Cards in a hopper or stacker (pocket) are shown as a portion of the area colored like plain punch cards. Most hoppers/stackers have a maximum display capacity of 100 cards. If more than 100 cards are in the hopper/stacker, a red line is shown on top. There is no actual limit to the number of cards that a hopper or stacker can hold.
Hoppers are generally located in the upper right, with cards flowing to stackers in the lower left. But each machine has it's own distinct card flow and direction. Hovering the mouse pointer over a hopper or stacker will normally pop-up a label showing the name of the hopper or stacker and information about the current contents.
Note that punch cards are not physically moved through the machines. The original file used to load the cards is never changed (unless you save some stacker over it). So, if you pass a deck of cards through a machine, then save the stacker contents into a new file, you have two copies of the same card deck.
Default behaviors for hoppers and stackers is as follows. Some machines may not support all of these, or may alter some of them.
Left-clicking on a hopper will normally start a dialog to pick a file (deck of cards) to add to the hopper. An option "Remove All" is normally present to remove existing cards first, otherwise the new deck will be added to the end. If the hopper contains blank cards, all black cards will be removed (in other words, it is not possible to have both blank cards and pre-punched cards in a hopper at the same time).
Right-clicking on a hopper will remove any pre-punched cards and add 50 blank cards. If the hopper already contains blank cards, then 50 more blank cards are added.
Shift-right-clicking on a hopper will empty the hopper (removing any cards, blank or pre-punched).
Left-clicking on a stacker will normally start a dialog to save the contents to a file.
Shift-left-clicking on a stacker will start up the Punch Card Viewer on the current contents, provided the machine is operating from the DataCenter application.
Right-clicking on a stacker will empty the stacker, discarding any contents (contents will be lost).