A Primary Medical plan will serve as your primary insurance coverage for any eligible medical expenses for the duration of your trip. A Secondary Medical plan, however, will act in conjunction with your primary (not travel-specific) health insurance, and may only cover expenses not otherwise covered by your health insurance policy.
For clarity's sake, both policy types generally use the same process - you will be responsible for paying your medical providers for any medical expenses you incur during your trip, and will then be able to file a claim and seek reimbursement from your travel insurance provider.
Essentially the terms “primary” and “secondary” are there to identify the order in which a claim will be filed.
If you purchase primary travel medical coverage, you will file your claim directly with the travel insurance provider. If you purchase secondary travel medical coverage, you will first file your claim with your health insurer, and will be able to file a claim with the travel insurer only for the expenses that the health insurer will not cover. This will often include deductibles, co-pays, or other out-of-pocket expenses. In essence, if your health insurance does not cover an expense (or any medical expenses,overseas perhaps), your secondary travel medical insurance will step in and serve the same role as would a primary medical policy, up to the policy limits.
Emergency medical coverage, regardless of whether primary or secondary, will provide similar coverage while on your trip. You will be able to seek reimbursement up to the policy limit if you require emergency medical treatment for an unexpected injury or illness during your travels.