basically...
Casting is where an alloy is heated to a liquid stage and poured into a mould where it cools and is then removed from the casting mould then machined. This is by far the cheapest process.
Forging is where an alloy solid billet is heated and then two dies are squeezed together at enormous pressure to form the initial shape which is then machined for final result
Semi forging is..
A billet is heated to its "Soft stage" then the soft material is injected into a cast die to form the initial shape which is then machined
Cast pistons are usually made of a "Cheaper" alloy (Lowest cost per unit)
Semi forged of a "Higher quality" alloy
Forged of a "Highest quality" alloy. (Highest cost per unit)
you will find especially in subarus pistons will be labeled or referred to as Hyperutectic.
This piston is die cast of a higher alloy when compared to cast pistons. But is of a lower quality material than "Forged" pistons.
so the term used frequently being "Semi forged" is incorrect. these pistons are "High temperature die cast"