It mean that if we try to Power-Up to +3, we will receive a Sub-Stat! But with that kind of Rune. Now we can see the message: Power-up Bonus Properties Here's an example of a Rune Upgrade with a Rune Rarity of Magic to demonstrate the Power-up Bonus Properties of a Sub-Stat: Each Rarity give randomly a number of Sub-Stat! Higher is the Rarity the more Sub-Stat it will receive and of course the value of the Rune will raise! Additionally, the Rune change in his Rarity each 3 Power-Up: 3/6/9/12, and will receive randomly a Sub-Stat, but there's a exception, if the Rune is already in the Rarity when it Power-Up, it will instead receive a update in the Sub-Stat already present!
The primary slots 2/4/6 are commonly used for HP /Crt Dmg /Atk /Spd, and slots like 1/3/5 can only hold Atk+ /Def +/HP +. Here become currently 6 rune slots currently for every monster. When we want to rune our monsters, we can only equip specific rune types on each slot. Rune are items that your may power up and supply over your monsters to make them stronger. Summoners War - Rune - RarityThere's 5 different Rune Rarity! Each Rune, when created, will receive a Rarity. However, there are also a few ones that we can only craft, like Fight or Tolerance runes.