According to Richard Stallman at

This license can be used as a free documentation license. It is a copyleft free documentation license provided the copyright holder does not exercise any of the "LICENSE OPTIONS" listed in Section VI of the license. But if either of the options is invoked, the license becomes non-free.

This creates a practical pitfall in using or recommending this license: if you recommend "Use the Open Publication License, Version 1.0 but don't enable the options", it would be easy for the second half of that recommendation to get forgotten; someone might use the license with the options, making a manual non-free, and yet think he is following your advice.

Likewise, if you use this license without either of the options to make your manual free, someone else might decide to imitate you, then change his mind about the options thinking that that is just a detail; the result would be that his manual is non-free.

Thus, while manuals published under this license do qualify as free documentation if neither license option was used, it is better to use the GNU Free Documentation License and avoid the risk of leading someone else astray.

Please note that this license is not the same as the OpenContent License. These two licenses are frequently confused, as the OpenContent License is often referred to as the "OPL". For clarity, it is better not to use the abbreviation "OPL" for either license. It is worth spelling their names in full to make sure people understand what you say.

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