Hm. It's where you change dresspheres. Basically, press L1 during battle and select the dressphere you want to change to. In the menu you open on the field with the triangle button, there's a button saying 'Garment Grids'. Select it, there you can see the Garment Grids you've collected, see their effects, put and remove the dresspheres from them, etc. Hope I helped. ._.
Currently questing for all Blue Bullets of Final Fantasy X-2.