Guild Help File:
The bond between an Angel and its Guardian Spirit is very powerful. Each spirit has a unique effect upon the Angel whilst in the mortal realm, and focusing on this discipline will heighten that enhancement to its utmost potential.
Melioratio is one of three disciplines available to every Guardian Spirit. Each spirit has to ability to enhance the angel's attribute most closely associated with that spirit. So far it appears you can only enhance one attribute at a time, but the possibility exists that at higher stations more than one stat can be enhanced. It is something worth keeping in mind.