It is a particular type of Pure which wasn't created by the simple process of advancing skills, but instead was the result of finishing Nature Spirit quest. For those who have 13 Defence, they are usually aiming to complete Recipe for Disaster quest to OSRS Gold acquire Adamant Gloves, which offer high Attack and Defence bonuses.
Furthermore, 13 points in Defence allow them to wear Black equipment as well as a Slayer Helmet improving their defensive skills further. It enables them not only to be more dangerous in PvP, but also to be more efficient in PvE combat. This allows them to maintain their account and train non-combat related skills. It's worth noting the fact that 13 Defense Pure usually has to sacrifice a several levels in Attack to have a lower Combat level.
Rune Pures account have 40 Defence, which is the minimum requirement to equip items from the runite Tier. They are also able to wear Dragonhide Body and Fight Torso for a boost in their defensive skills. 40 Defence is also the standard requirement for the Lunar Diplomacy quest, which lets players play Vengeance as one of the most useful PvP spells available in the game. The f2p community typically favors this particular build as Rune Armor is the best one they can equip anyway. However, members on the other hand, are more likely to opt for Void or Berserker build.
Sometimes referred to as a Zerker or just commonly referred to as a Zerk. It is a variation of Rune Pure which utilizes 45 Defence to give players an option to choose between Berserker Helm, or Fighter Hat. Because they give better bonuses in comparison to Rune Helm, these additional 5 levels of Defence are preferred. Most Zerkers are also able to Buy RS Gold complete Recipe for Disaster for Barrows Gloves, which are the finest slot gloves on the market.