Steam a seam is a fusible used for applique, fuses on both sides, fused to wrong side of one fabric, paper peeled, fused to right side of background-- is a common application. ( washes away when laundered)
Fusible interfacing has fusible on One side, stabilizers fabric -- stays in, comes in various weights, lightweight used in light fabrics, silks, etc. Medium or heavier weights used for things like stiff collars, cuffs, where stiff stability is needed. They ( SaS & interfacings) are two very different products.