What does the score mean in campaign missions?

Solution 1:

The symbol is a medal (3 yellow cubes for gold, 2 grey for silver and 1 reddish for bronze).
The number is also how well you've been doing but Mode 7 had no time to tell me what determined it nor the medal. It seems to be different for each mission type and is not always consistent.
Also the high score shown in the main window with the seed etc is supposed to be your best score but it's not at all cause it's bugged.
In relation to this you might want to have a look at the Frozen Synapse Campaign Missions Records to compare scores and try to find the formula (why not).