In the ASME Y14.24 context, any drawing that is not identifying. Ancillary drawings are most often used to document Engineering information that is better understood in graphical form (as opposed to textual), but is not mandatory for the manufacture of the subject part.
This concept can confuse Engineers who do not have a strong design background. Examples of ancillary drawings include schematics, installation drawings, and layouts. At least a quick read through ASME Y14.24 is strongly recommended for all Engineering staff.