In the NFA world, "post sample" means that the machine gun was built AFTER 1986. Therefore, it cannot be owned by a civilian. Post sample machine guns are only allowed to be owned by class 3/SOT holders. The purpose of having a post sample gun is for demonstration by a class 3 dealer to a law enforcement or military branch seeking to purchase machine guns.
Examples of post sample machine guns

• HK 416D
• HK MP7
• HK UMP
• FN Scar
• Modern AR-15 (fully automatic)

On the other hand, transferable means that the gun was added to the NFA registry before the 1986 cutoff date. Therefore, it can be sold and transferred for civilian ownership. Transferable guns are also called pre-ban guns. See more information on owning pre-ban machine guns.