The paintings will be classified according to their subject and our guests will have the chance to see them and talk with the creators.

classified 是动词 classify 的过去分词形式，在句中表示被动。classify 由词根 class（类别、等级）和后缀 -ify（使...化）组成，意思是"分类、归类"。classified 在这里表示"被分类的"。