what does OP mean?

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You would have to order the device as 88NL less trim, the device will be shipped as an 88EO.  The EO device has an NL cam to accept a rim cylinder tailpiece. 
The NL-OP function for 25-C uses the 718C trim, so template T-2211 for metal doors and W-2211 for wood doors for 718 trims prior to 2/28/2011. T-2211 for trims prior to 2/28/2011 W-2211 for...
Device would be ordered as a 25C-C as of June 2012.
This option is not available as there is no NL cam to directly drive the rods and using a key to lift the heavy stainless, brass or bronze rods would damage the key. 
No, the 98/99 series mortise exit device should be ordered as NL less trim. IE, 9875NL-L/TRIM. NL-OP suffix is normally used with the 98/99 series rim, surface and concealed vertical rod devices wh...