Arms Plant is where you train your units. There are in total 16 types of units. You can build multiple Arms Plants in a city. Each Arms Plant can only train 1 unit at a time. However, you can put more units in the training queue, so they will start being trained automatically. The level of the Arms Plant is the pre-requisition of the training of higher level units. It also determines the capacity of the training queue. If you own multiple Arms Plants in a city, you can also use "Evenly Assign" to automatically coordinate the training queues so they will be shared between all available Arms Plants.