[請教]LEE soft ND grad (100x150mm) 0.9 + 0.6 的意思

在 google+ 上看到 Patrick Smith 這位攝影師拍攝的照片
https://plus.google.com/u/0/102871123574468799566/posts

每張照片會列出拍攝的詳細資料，如

Details

Canon 5D Mark II
Canon 17-40L @ 20
1.4-second exposure @F11
LEE soft ND grad (100x150mm) 0.9 + 0.6
Lee foundation kit filter holder with Lee 77mm adapter ring
ISO 100
No polarizer
RAW file processed with Capture One by Phase One
TIFF file processed with Photoshop
Small Slik Sprint mini II tripod
Manfrotto 322RC2 pistol-grip ball head

常看見他使用

LEE soft ND grad (100x150mm) 0.9 + 0.6
或
LEE soft ND grad (100x150mm) 0.9 + 0.9

LEE soft ND grad 應是 LEE 的 漸層濾光片
那 0.9 + 0.6 的意思是指
用了兩片，一片0.9 + 一片 0.6
還是濾光片本身就有種規格叫 0.9 + 0.6

感謝