All elements in the base DITA DTDs have preassigned DITA2Go properties, listed in §B. Element type default properties. You need to add or modify property assignments only to accomplish one of the following:
For the former, as an example you might want to use the value of a particular element as a variable; see §12.1 Understanding how DITA2Go user variables work. If that element does not already have property Var by default, you would need to add Var to the list of properties for the element.
For the latter, if you do not specify properties, a specialized element inherits the properties of the element from which it was derived.
If your DITA document includes specialized elements with presentational characteristics different from those of the base elements from which they are derived, you might want to assign appropriate properties to the specialized elements.
On the other hand, if you convert specialized documents without defining element type properties, and you like the results, you need not specify these properties for your specialized elements. That will happen if you have specialized from base elements that have the same presentational characteristics as your specialized elements. This is the DITA process for generalization, which is what makes specialization so useful: tools that do not understand a specialized element are required to treat it as the highest base class they do understand. That handles most cases.