大毘盧遮那成佛經疏 10.185

10.185

右六字，如上所說十佛剎塵數菩薩種子也，隨用一字即得通用之耳。凡諸真言中，隨取初中後字用為種子，皆得也（觀察取宗要之字耳）。

Above are the six syllables, the bījas of the bodhisattvas numbering as many as the dust motes of ten buddha lands as mentioned above; using any one syllable will serve universally. In all mantras, one may take the initial, middle, or final syllable at will to use as the bīja (observing and selecting the syllable of essential doctrinal purport).