Note to self: After merging, the following must be added to https://wiki.freecad.org/NativeIFC under Classes, attributes and properties:

Custom properties and default property sets can be added either by right-clicking the properties editor and choosing Add property... or, in FreeCAD 1.01, using the two buttons below the properties editor: insert image here

Properties added with the two above buttons will have their IFC type saved with the property. For other properties added with the right-click menu, there is no IFC information saved, and, when exported to IFC, the IFC type of the property will be decided based on the conversion table found at insert link here. That table define the conversion between IFC properties and FreeCAD properties. If you have imported objects from an existing IFC file, though, or if you created the properties using the two buttons above, this conversion is only visual, and the right property type will be saved within the IFC file.