DynamicData is an external workbench with which one can create a container object for holding custom properties.
With this workbench it's possible to create a new custom property of just about any type that FreeCAD supports. For example a Length property or a Placement property. These custom properties can then be used in Expressions just like any other property. For example, you could create a Length property called "Width" and refer to it when constraining a sketch element. Then when the "Width" property is changed, the sketch constraint will automatically update. This is similar to the way one can use a spreadsheet, but is more interactive in that the properties can be changed while still being able to see the 3D view and also allows for a wider variety of property types.
Some features include:
This workbench can be installed from the Addon Manager. For manual installation see Installing more workbenches.