佛頂尊勝陀羅尼經教跡義記 2.173

2.173

述曰：第十三、安置處所。有其四處：一、幢上；二、山上；三、樓上；四、窣堵波中。安置此四處，有其二意：一、高顯；二、清淨。

The commentary says: Thirteenth, the locations of installation. There are four places: first, on a banner; second, on a mountain; third, on an upper floor; fourth, inside a stūpa. Installing it in these four places has two purposes: first, elevated and prominent; second, pure.