What describes packages sold commercially that can be customized by the developer?

The term that best describes packages sold commercially that can be customized by the developer is Custom Off-the-Shelf Packages. This refers to software solutions that are readily available for sale (off-the-shelf) but are also designed to allow for customization to meet specific user needs or business processes.

This flexibility distinguishes these packages from purely generic commercial software, which may not offer customization options. Custom Off-the-Shelf Packages strike a balance between the ease of deployment associated with commercial software and the tailored functionality that businesses often require. As developers can modify these packages, they can adapt them to fit unique workflows or integrate additional features without starting from scratch.

Other terms such as Control Systems and Data Validation Tools do not convey the concept of customizable commercial software, as they refer to specific types of software with limited scope and functionality related to their respective fields.