A health insurance plan is considered a Marketplace plan if it was purchased through a state, federal, or private Health Insurance Marketplace (including Healthcare.gov or a state Marketplace/Exchange). Marketplace plans are commonly called Obamacare.
Health insurance plans provided through your employer are not Marketplace plans, nor are plans purchased privately from a non-Marketplace.
The plan administrator can help you determine if you have a Marketplace plan if you are still in doubt.