Let's look at the slots and what the difference between "The short ones" and "the longer ones".

           
               01               45   48  58 --Side A
              +--------------------+-------+  (component)
REAR          | ================== | ===== |              FRONT 
              +--------------------+-------+  (soldering)
               01               45   48  58 --Side B

16-bit MCA slot with AVE "8514/A Slot"

           
       V10 V1  01               45   48  58
      +-------+--------------------+-------+
      | ===== | ================== | ===== |
      +-------+--------------------+-------+
               01               45   48  58

32-bit MCA slot

               01               45   48            89 
              +--------------------+-----------------+
REAR          | ================== | =============== | FRONT
              +--------------------+-----------------+
               01               45   48            89

32-bit MCA slot with MME "Memory Card Slot"

             MM 1               45   48            89 
           +-----------------------+-----------------+
           | ===================== | =============== | FRONT
           +-----------------------+-----------------+
             MM 1               45   48            89

32-bit MCA slot with MME and BVE "Video Slot"

    V10 V1    MM 1               45   48            89 
   +-------+-----------------------+-----------------+
   | ===== | ===================== | =============== | FRONT
   +-------+-----------------------+-----------------+
             MM 1               45   48            89

All views are from the top looking down to the slot
and are not exactly in scale.

The 'component' and 'soldering' referres to an
installed Adapter-Card.

AVE = Auxiliary Video Extension
BVE = Base Video Extension
MME = Matched Memory Extension

AVE = Auxiliary Video Extension

The AVE part of the MCA connector consists out of 20 pins (2 x 10) at the rear end of the MCA connector, separated by a key (notch in the cards' connector).

The AVE is intended for cards that have no own base-video system - like the IBM 8514/A Display Adapter /A.
This adapter can utilize the planar video-system through this connections and the video-signal in the low-resolution modes come in fact from the onboard-VGA / onboard-XGA, but is passed-thru to make it available on the 8514/A monitor port.
Therefore you need only one monitor.

Other additional video-cards that *do not* use the AVE cannot display the VGA text/graphic mode on the same screen along with their own native modes. They need a second monitor attached to the onboard VGA / XGA card.

In a way this AVE is the MCAs Video Feature Connector: cards can use the onboard video-system and -for example- syncronize their output to the onboard video.

MME = Matched Memory Extension

The MME part of the MCA connector consists out of 8 pins (2 x 4) at the rear end of a 32-bit MCA connector. There is no separation between the 32-bit connector and the MME-part.

The MME part is used from 32-bit Memory Expansion adapters, which support Matched Memory Cyles. This can improve the data transfer capabilities between the system master and channel-resident memory (means: between planar memory and that on expansion cards).

The use of MME is system-dependent and varies between the line of PS/2-machines.
The Models 70 and 80 support MME - and therefore can be upgraded with a 32-bit memory expansion card, the Models 90 and 95 do not support MME and should be upgraded only with memory on the planar.

BVE = Base Video Extension

The BVE part of the MCA connector consists out of 20 pins (2 x 10) at the rear end of a 32-bit MCA connector and after the MME-part. There is a separating key between MME and BVE, which causes a notch in a cards' MCA connector with a size of 2 connecting pins.

This slot can only be found on machines, that do not supply an onboard Base-Video system, like the Models 76, 77, 90 and 95.
The machines' video card (XGA / XGA-2 or SVGA) plugs into this slot and is enabled to communicate with a card which sits in a probable AVE-slot. All sync-signals, blanking and pixel-data is supplied over this BVE connector.
On the XGA-adapter the BVE-connector is the short one to the rear, the MME-part is left out, therefore there is a very large gap between the BVE-part and the rest of the MCA-connector. If the XGA is set to boards with Base Video this BVE-part plugs nowhere and is just hanging free, because systems with onboard-videocard don't have an BVE-connector.