Are subscriptions Product ID unique per app or per developer account?

Hello,

I'm trying to create a subscription for one of my apps.

When I specify the Product ID yearly_3, I get the following error message: The Product ID you entered is already being used by another subscription.

This app does not have any other subscriptions with this Product ID. But I have another app with a yearly_3 Product ID.

According to the official documentation, the Product ID is A unique ID specific to your app. So I thought it was possible to have the same Product ID between apps, but not for the same app.

What's the rule here?

Thanks,

Axel