PSE is timber that the edges have not been rounded in any way, with each edge having been planed. It is an alternative to rough sawn timber with the caveat that PSE has had each edge planed perfectly straight, to a smooth finish.
At Supreme Timber we have a 2″ x 1″ redwood PSE. An excellent product for contemporary fencing, feature walls and many other DIY projects. The redwood PSE is available in tanatone brown and tanalised green.