[Suhiir]

In a campaign you have to select ALL the CORE units to be used and keep that part of the OOB unaltered throughout the campaign (the game itself will keep track of what's damaged/destroyed). You can add "auxiliary" (i.e. they only appear once as "Auxiliary" (ONLY) units, and of course you can do what you like with the OpFor.

To keep CORE units from appearing in a given mission just set them as reinforcements scheduled to arrive on some turn greater then the scenario length, that way they never actually show up. UNLESS they're artillery, artillery will still fire counter-battery missions, I often use this to represent air strikes the player has no control over vs OpFor off-map artillery.