The two buttons in the datum section are intended for the quick addition of datums to a part. Press the Datum letter button and then click on the part to define the tie-in point for the leader line used by the annotation and to select the associated CAD faces. The Datum letters increments automatically to the next available letter. To enter a letter that is not in sequence, hold the letter you wish to use down on the keyboard when you press the datum button. It will then add this selected letter when you click on the part. Datum letter utilization is controlled by the selected GD&T Standard. ASME does not allow the use of the letters I, O, and Q, so they are excluded in the sequence.
Double or great datum letter designations such as AA or AC can be created and are recognized in SA. However, they cannot be built through the toolkit. You can build them using only one letter “A” for example and then open the annotation properties and change the name after building the annotation. Another alternative is to use GD&T> Make Annotation for GD&T Datum to build a blank datum annotation, add the Name designation desired and then associate the feature object.