The armor can hold a number of Spells or Techniques equal to its Tier. A Rune Mage can add a Spell or Technique they know to the armor with 8 hours of work (removing another spell from it in the process if needed to make room for the new one), along with one Metamagic they know for that Spell or Technique.
While you are Attuned to and wearing the armor, you can cast any Spells and Techniques inscribed on the armor as if you knew them, casting them and adding Metamagics to them normally. The only Metamagics you can add to a Spell or Technique are those inscribed with them on the armor or ones you gained via a Keystone granting them to all spells of a certain type.