the worker pool.
a worker is a yellow token in your play area, but not in your yellow bank.
a worker that is not on a card is an unused worker.
a worker on a technology card is a farm or mine, an urban building, or a military unit. it has the same level and type as the card.
you can move an unused worker to a card with a "build" action. you pay a resource cost.
you can move a worker from a lower level card to a higher level card with an "upgrade" action. you pay the difference between the two resource costs.
you can remove a worker from a card with a "destroy" 