In all current forms of the game since AA Anniversary edition, subs (and transports) can not and do not ever block movement. So if in your example you can attack a stack of subs with a stack of destroyers and other units are free to proceed through to another seazone, even though there will be combat in that seazone (destroyer on sub).
Transort Question
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In A&AR is a transports capacity 1 inf and 1 land unit or 1 land unit and 1 inf? What I mean is do you need to have an Infantry in order to transport a tank or can you just load 10 tanks on 10 transports?
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Capacity is UP TO 1 ARM/1 INF or 1 INF/1 ART or 1 INF/1 AA or 2 INF.
You can TRN any lesser amount you choose, you just cannot exceed the above limits.
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But why send the armor without fodder to take the hits? You can, but why would you?
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Sometimes you only have one unit to move to somewhere else SSG Rock.
If you have 10 ARM and want to transport them all on the same turn you will require 10 transports. You may load those 10 TRN with 10 INF as well but as mentioned you are not required to do so.