ST_OnOff is used extensively in the XML Schema. Here is the openiso.org link (nice resource!).

Basically, it is used for things which should use the built in boolean schema data type:

This simple type specifies a set of values for any binary (on or off) property defined in a WordprocessingML document.

For example, the b (bold) element has an attribute @val of type ST_OnOff.

There are several problems with how this is done.

The first is that its possible values are “on, 1, or true”. OOXML should just use the XSD boolean data type, which doesn’t allow “on” (or “off”). For related comments, see here, here, and here. Denmark and France seem to be the strongest advocates of the use of xsd:boolean, and I hope they get their way.

The second is that it is left to the specification text to say that if the attribute is omitted, its value is implied to be true. That should be expressed as part of the schema.

For CT_OnOff, it would be:

<xsd:complexType name=”BooleanDefaultTrue”>
<xsd:attribute name=”val” type=”xsd:boolean” default=”true” />
</xsd:complexType>

I don’t think Denmark or anyone else made this second point.

The schema we are using in docx4j to generate classes uses these sorts of definitions instead of ST_OnOff or CT_OnOff.  For CT_OnOff, this results in a BooleanDefaultTrue type, which is used in fields like (for bold):

protected BooleanDefaultTrue b

Which brings me to the the third problem with ST_OnOff (and the schema in general), which is that it generates ugly code in JAXB and other binding frameworks (presumably .NET included). The built in schema data types produce much nicer code.

As a general remark, running the schema through JAXB is a good way to find places where the schema can be improved. Schema design goals should include:

1. that it can be processed out of the box by binding frameworks (since that makes it easier for people to pick up a schema and start using it). [This is not currently the case]
2. that the schema be expressed in such a way as to generate the simplest code.