Pressure treatment involves putting timber into a pressure vessel and immersing it in perservative at high pressure to ensure that thechemicals penetrate deep into the timber.  This can considerably extend the useful life of timber, especially in locations prone to fungal or insect attack (eg fenceposts, cladding).  The logistics of pressure-treating your ready-cut timber are considerable, and you will need to discuss this with the ASHS member concerned.  ASHS members offering this service will also sell their own timber which has been pressure treated.