There are 2 circumstances, and the trigger varies depending on each.
1) "Money" system, bring a certain number Civ to a location, remove those Civs, and create a new unit at a new location.
2) "Buy" system, bring a unit to a location, move the unit back to a nearby location so you won't buy more than you want, and create the bought item at a new location.
Ignore the people who told you to use switches, they're just being redundant.

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But hey, using switches allows you to move the "buying" unit back and forth across it without creating more than one. Just don't leave the unit there or a new unit will spawn every few seconds!