The rune platebody is a platebody made of runite. Completion of Dragon Slayer is required to buy this item from Oziach and to equip it. Statistically, it is the second best non-degradable melee chest armour available for non-members, behind the Gud raider chainbody, which has the same defensive stats and an additional strength bonus.
Prior to the Personalised Shops update, members could make a profit by buying the platebody at the Blast FurnaceArmour Store for 45,000 coins and then selling them at the Grand Exchange for around 70,000 coins. Currently, the Armour Store carries no stock of its own.
While the dragon platebody only requires level 92 Smithing to make, the rune platebody requires level 99.
Aside from gameplay reasons, this could be explained as simply reforging the dragon platebody from preexisting pieces requires less expertise than creating a new platebody from a hardy metal such as runite from scratch.