What is a 'craft backer' on a roll of flashing? Is it an adhesive? Another layer of metal? And what does the 3-ounce unit refer to in this context? Is it analogous to gauge?
... laminated copper flashing with a 3oz craft backer that [can be used] with pressure treated lumber
"Craft" (or kraft) paper is a heavy-weight material used on building products, often insulation, as a barrier for various purposes. The name refers to a wood pulping process used in its manufacture. In this case, it would serve to isolate the metal from a chemically harsh lumber treatment. For insulation it can act as a containment layer and prevents moisture migration.
"3 oz." refers to the specific paper weight (per unit of area--probably a yard). It gives an indication of relative thickness. It is a gauge of sorts.
