Hey Boudewijn,

The creation of columns is now centralized in the Data model. Since expression fields are part of the data model and are very similar to 'normal’ columns, it is handy to put them together. It is also possible to create references between tables by using expression fields. 

If these expression fields would be instead created at Subjects, it would cause us to have two places for Columns to be created. Besides that, I think the reference aspect would then be utilized less.