Why can some models in the gallery be ordered, and others not?

Web users who design 3D models can choose between different options when they upload their files to the website, in the sharing part of the form:

- public: the model is visible by all users and can be ordered

- visible: the model is visible by all users, but can't be ordered

- private: the model is only visible to the user who uploaded it, in the "Account/Your designs" section

- for sale: the model is visible by all users and can be ordered. The designer will be paid a royalty fee, based on a percentage of the 3D print order of their design