When the game begins you will only have one city (normally) and no units. Every city you own can produce units or supplies for you. One of the major challenges in this game is to produce the units you will need later. Because production takes time, you must anticipate how the game will progress.
In addition to the time needed to produce a unit, supplies and technology levels may limit your production. (If you do not want to deal with this complexity you can use a rule set in which supplies and technology levels are not used.) Supplies, such as money, steel or oil must be produced and will be used up to produce units. Technology levels, such as industry or science must reach certain levels before some units can be produced, but are not used up. Both supplies and technology levels can be local or global. The local levels apply to one city and cannot be moved. The global levels are available for every city you own. In addition a local supply or technology level may be captured or destroyed when a city is conquered. Hence there are six distinct categories.
The names of all supplies and technology levels can be changed so this explanation may not apply. A local supply represents something like food or concrete that is bulky and difficult to move. A global supply represents something like money or gold which can be moved almost arbitrarily.
A local technology level represents factories and shipyards in or near a city that cannot be moved. A global technology level represents scientific discovery or organizational ability.
In addition to limiting which units can be produced, technology levels also can affect the abilities of units and their cost. A player with a very high technology level might be able to produce a faster fighter or more powerful ICBM.
