Product portfolio roadmaps are strategic plans for multiple products.That's why they're often also called multiple product roadmaps. They often visualize long-term periods of time of about 9 month up to 5 years. The further items are placed in the future the less detail it includes. A portfolio roadmap works perfectly for teams or organizations when they have a broad product line and want to visualize a high-level summary across all of their products.
On the value side, product teams often consider economic impact, competitive advantage and customers whilst prioritizing the items that make it onto the roadmap and rate these against development and operational costs.