Short answer
If your company is doing business in a state other than where it was formed, that state usually requires you to register as a foreign entity, called foreign qualification. What counts as doing business varies, but having employees, an office, or regular in-state operations typically triggers it, along with appointing a registered agent there.
Foreign qualification is how a company formed in one state gets permission to operate in another. The home-state formation does not by itself authorize you everywhere. States look at whether you have a physical presence, employees, or ongoing business activity within their borders to decide whether you must register.
Qualifying means filing with the new state, paying its fees, appointing a registered agent there, and then keeping up with that state's annual reports. Occasional or purely online contact may not trigger it, but a real operational presence usually does.
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