The deck is the surface where the head is bolted too.
The open deck design is for ease of manufacturing and not good for high cylinder pressures. This cylinder wall design is supported in 2 places. Many people will say that these engines cannot be turboed effectively but that is not the case. There are a couple here in the club that have run low to medium boost without issue. The problem is where the top of the cylinder where it meets the head will distort from high cylinder pressure and damage the head gaskets ability to seal in the combustion gases.
A semi closed deck design is a little harder to manufacture, but supports the cylinder at the top, bottom and sides. (four places). This is stronger then the open deck design and therefore can take higher cylinder pressures.
The fully closed deck design uses a different casting process that takes longer to manufacture and weighs more. These blocks have the most support to the cylinders, which will enable you to run the highest cylinder pressure of the available choices.
Subaru introduced the closed deck block for homologation reasons to be able to run it in their rally cars. This engine was made famous when a very similar version to it was installed in the limited edition 22B. With the casting methods and alloys available at the time Subaru chose this route to ensure a strong motor. With todays “modern” casting methods and alloys even the semi closed deck designed motors are very strong.
As a guide I would say the following;
Open deck blocks will work for all NA or low to medium boost applications.
Semi closed deck blocks are better suited for boost applications that are not extremely high or prolonged.
Closed deck block are best suited for applications where there will be very high boost for extended periods of time.
Good read - understood abit more about this open-semi-closed deck business.