I posted this a few years ago. It may be useful:

A short lesson: "High function" video adapters such as the 8514/A and
XGA can work in conjunction with existing VGA adapters. On the Micro
Channel bus, there are 2 types of video connector: Base Video Extension
(BVE) and Auxiliary Video Extension (AVE). Any adapter with VGA
capability should have a BVE connector to feed VGA data to the bus.
(Systems with system board mounted VGA will also put their VGA data on
the bus). Any adapter without VGA capability (e.g. 8514/A) will have an
AVE connector to receive VGA data from the bus.  This is so that you can
plug your monitor into an adapter without VGA capability, and it will
"pass through" the VGA data when in VGA mode.

Now, how to tell the difference: You need a ruler. Look at a Micro
Channel socket. About half-way along, there is a "key" or bar that fits a
corresponding slot in the card connector. This is the datum point from
which all measurements are taken. All measurements are fron the centre
of this key to the end of the rearmost slot in the socket.

A slot which measures 58 mm (2.28 in) is a standard slot. (16 and 32 bit
slots differ in the length of the slot *forward* of the key.)

A slot which measures 64 mm (2.51 in) is a Matched Memory Extension (MME)
slot. The extra pins provide memory clocking signals to some models of
memory adapter.

A slot which measures 74 mm (2.91 in) is an AVE slot. (It will also have
a second key at the 58 mm (2.28 in) point).

A slot which measures 79 mm (3.12 in) is a BVE slot. (It will also have
a second key at the 58 mm (2.28 in) point).

An 8514/A will only plug into an AVE slot. It has no VGA function.

An XGA/A will plug into either a standard slot, an MME slot or a BVE
slot. It has VGA function.
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... and some more from another post:

Unlike ISA slots, Micro Channel slots start near the middle- at a "key" part
way along the socket. The key is mandatory, because it's possible (and
valid) to plug, say, an 8 or 16 bit adapter into a 32 bit slot and the key
is all that stops it moving backwards or forwards. The critical part is that
even if it's only an 8 bit card, it must have enough blank connector forward
of the primary key to ensure correct location.

If a card only has connectors between the key and the rear edge, it's an 8
bit card. If it has a short set of connectors forward of the key, it's a 16
bit card. If it has a long set of connectors forward of the key, making the
section forward of the key about as long as the section behind it, then it's
a 32 bit card. If it has 4 extra pairs of connectors at the rear end, it has
MME. If it has a second key at the rear and 10 more pairs of connectors, it
has AVE/BVE.
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