This is from the link that Buzz Box provided:
Summary
The goal of this Briefing Leaflet is enhance your understanding of the landing performance data and to appreciate the role that speed
brakes and thrust reversers play in deceleration especially when landing.
As a review:
Certified data is factored, used for dispatch purposes and does not use reverse thrust.
Advisory data is non-factored (FAA), used enroute and has always been based on the use of reverse thrust.
Speed brakes increase drag and decrease lift allowing your brakes to be considerably more effective.
Reverse thrust provides significant deceleration when landing on a critical runway; short, slippery with the braking action
reported POOR.
The bolding is mine. Can I suggest you download the document, have a good read then let us know if you still are confused.