It is the category of option page itself. It must be the same string as in ProvideOptionPage attribute on your package class:
[ProvideOptionPage(typeof(FooPage), "<THIS STRING HERE>", ...)]
class FooPackage : AsyncPackage
{
...
}
It is the name of option page itself, as declared in ProvideOptionPage attribute on your package class:
[ProvideOptionPage(typeof(FooPage), PageCategory, "<THIS STRING HERE>", ...)]
class FooPackage : AsyncPackage
{
...
}
The actual name of a property in your code, not anything in the attributes. For example:
[Category("Foo")]
[DisplayName("Bar")]
[Description("Baz")]
[DefaultValue(true)]
public bool Qux { get; set; } = true;
In this case the PropertyName is "Qux".