I dont know if there's an overview concerning game variables at another place within this forum. So - I'll present my modest knowledge here:
There are exactly four territory variables for each territory used,
there are exactly four company variables for each company used,
there are exactly four player variables for each player used,
but there are only four game variables at all.
The latter one are most simply to use, because when using the other ones you always have to specify "force test against ..." in the right way. Besides that, any variable may hold any value you want and you can do value assignments (e.g. random numbers) and limited arithmetics. It's up to you to do something reasonable with them
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Also, you should be careful when choosing a variable, if you want to present it's value within the status or a newspaper - they may not all being formattable as you like. I found that the game variables, the company variables as well as territory variables 2 and 3 allow different presentations (numeric, money, boolean [e.g. true or false], connected and completed), whereas the remaining ones only appear as number.
@Markman: If you want to present e.g. the "number of loads of rice hauled from territory Xyz" as part of the status, you simply have to create a separate status event and specify "force test against territories", so that you can choose the one you need. I did this twice in my 'Rorschach-Heiden Mountain Railroad' concerning "beer from Rorschach" and "meat to Heiden".
BTW - as a programming mathematician I enjoyed the discussions above concerning random numbers!
There's no business like RT business ...