When you add a field to a form template, you can select the data-type for that field. We support over 15 different data types, for example Text, Numeric, Checkboxes, Map, Dates, etc. Each of these data-types is stored differently within D4H, and as we've enabled more powerful functionality such as conditional expressions, dashboard stats, header modules, and custom views - each data type has taken on more abilities.
Recently, our Help Desk discovered that a number of issues customers were encountering were occuring due to the 'Data Type' of a field being changed, after logic was applied to it and data had already been added.
To avoid this, we've restricted editing of the 'Data Type' on a field. You can still change all of the other properties, but if you decide for example decide to move from a Text field to a Drop Down, you'll need to delete that field and re-add it.
There's a guide here in the Knowledge Base.