The answer to that is tied to a discussion about displaying the posters so the customers could see them. Most stores did not have enough room on their walls to mount all the large Nike posters. So, as a means of advertising NIKE adopted a 5 x 7 inch format and printed "mini posters", if you will, in order to properly display them. NIKE went through at least 3 generations of counter and floor display models as shown below. Fellow collector Ortreasures worked in a printing company in Portland at the time and recalls that these were indeed called "poster cards" by all those involved in their production.
Generation 1 Display
As discussed in blog 1 customers were initially sent a box of poster cards with holed punched in them. They were told that the "most popular" poster could be prominently displayed in the one slot in the acrylic display and the rest could be attached to the base of the display by threading the plastic fastener through the cards and the hole in the acrylic display as shown in the photo.