According to the @SethDu here, "In terms of Standby server, the quantity of it should be larger than 1 but less than the Maximum, it simply shows how many servers will be in stand-by states. The advantages of it is that players are able to quickly access to the multiplayer server otherwise they will wait for the server to boot up and get ready."
However, when either creating our build through the CreateBuildWithCustomContainer API call, or modifying an existing build through Game Manager, the number of StandBy servers never seems to decrease and always is set to the maximum.
Here's an example: We've created a build in EastUs with 4 maximum servers and 1 StandBy server (in production the number of StandBy servers would be way higher, but still not the maximum, but we'll use 1 as an example).
Notice that at the top, we choose 1 Standby server in our configuration. However, at the bottom is the view from the Servers page in Game Manager showing that there are currently 4 servers in Standby mode.
If it's a simple non-bug issue and I'm misunderstanding I apologize!
My best guess is that it is one of three possibilities:
1. The Game Manager is wrong/poorly worded and there is actually only 1 server in Standby and the rest haven't been allocated yet?
2. I'm misunderstanding the concept and Standby servers refers to VMs instead of containers within the VM. So, for example, if we have 4 servers per-VM with 8 max servers per region, if Standby was set to 3: one VM would be active (4 servers) and one wouldn't?
3. There is a bug and Standby servers are always allocated even if the number is less than the maximum?