In order for a design to be complete, tolerances must be specified for all dimensions such that the limits of variation are defined for all features in the design model. This message is shown whenever the model contains one or more dimensions that are shown as 'nominal' (i.e., tolerances are not explicitly specified).

ASME Y14.41-2012 (par. 9.2.2) includes a model requirement that 'a displayed feature of size dimension shall always include a tolerance' so showing a size dimension as 'nominal' is not allowed.

There is no similar statement in ASME Y14.41-2012 pertaining to other dimension types such as offset or angular dimensions. Although It is generally considered good practice to explicitly specify tolerances for all dimensions, it is not required for non-size dimensions. Whenever you have a model that includes 'nominal' dimensions, this message will be displayed and you should express tolerances for those dimensions in a general note.