Chapter 19 of the 4DBK Language Reference Manual discusses the fields used in 4D Business Kit. The fields are divided into six categories: Item Related, Basket Item Related, Identified Customer Related, Order Related, Store Related and Other. Each of these fields has many attributes. These attributes define how that particular field can be accessed. This listing only applies when the fields are accessed through the Web http interface.
For example, if you want to update a field of an Item Related category, you would not be able to do so through a browser. The "U" which stands for update is not active. Instead, to update any field in which the "U" is not active, you can do so through the 4DBK IP Component.