Ok so how realistic this game is expected to be?
As in who will supply the weapons as when arms dealing is concerned mostly russia and USA coems to mind and its an even smaller scale when you play as a small time private arms peddler who likely won't get his hands on export variants of T-72s... like ever.
Or will it be a more "alternate history" setting where PMCs can roll into let's say Poland and order 12 PT-91Ex assuming their standing with Poland and cash balance is high enough.
Most of this information is available in the other posts already but I am happy to sum it up for you. There are 10 supplying nations that a player buys from, these are the current 10 top arms trading nations in the world
1 Russia
2 United States
3 China
4 France
7 United Kingdom
5 Germany
6 Italy
8 Israel
9 Spain
10 Sweden
Ukraine was actually #10 but I removed it for the sake of gameplay since they mostly only export Radar and electronics equipment and the small arms that they export are the same as Russia and the US. Sweden on the otherhand has some really cool weapons. Realism isn't a super high priority in a game because it's not very fun. In fact when you design a game for the first time you get to experience a life changing event(for gamers) where you very clearly see that adding or removing a realistic element almost always directly impacts the fun experienced by the player. You need to include enough to create immersion but remove enough to be fun. In my experience it is best to include realism in the background and elements not under player control and let the player break reality with their actions. It is a fantasy they want to believe in so they are able to avoid breaking the illusion. Deep down we all want to be powerful and have the rules apply to everyone but us, so when designing a game it's important to play to that psychological desire in limited doses.
The short answer, it's set roughly 10 years in the future so it's not necessary to make it alternate reality, but it doesn't necessarily preclude the concept that it could be. If I do my job well, you won't ask that question while playing.