What does it mean for bitcoin to be legal tender in El Salvador?

Does it mean all shops are forced to accept bitcoin or shops can accept bitcoin only if they want to? Logically, it doesn't make sense for it to be the first option because not all shops will have the infrastructure to (e.g. a small convenience store or grocery shop). If it is the second option, then I doubt many shops will opt for it.

Or does it mean products in El Salador are priced in satoshis?