When you add a layer to ArcGIS Pro, ArcGIS Pro will automatically define a Display Field (if the layer contains a suitable field). This could be for instance the first text field (not entirely sure what logic is applied to define this field). You can set it manually in the properties of the layer:
When you activate labeling for a layer, by default the Display Field is selected for labeling. You can change this to another field if you want.
Thank you. I appreciate the answer. You have confirmed about all I know about the Display Field - that once assigned it becomes the default Label Field. Beyond that I have not seen what relevance it has for the Feature Class it belongs too. I was wondering if it had any other influence on the FC. ESRI documentation seems weak on the subject.
Another usage is when you have a relate in your ArcMap or Pro session. It will be used to show the related features in the identify window. Using an expression in that case enhances the readability of the identify results.For instance concatenate a code that is not unique with something (perhaps a measurement) that makes it unique:
Just stumbled upon some more information here: Understanding the display expression for a field—Help | ArcGIS Desktop
The display expression defaults to the first field of string type that contains the text name (case insensitive) in the field name. If no name field exists, the display field defaults to the first field of string type, then the first field of integer type, then the first field of any type
... and some more info in this old Help doc: ArcGIS Desktop Help 9.3 - Setting field properties, aliases, and table display options
Rod.... there are a whack of error messages if you chose the wrong type
which would have been my first choice!
I suppose it has something to with publishing to web stuff I am suspecting from the errors... mine always defaults to OBJECTID, but I don't care much since I rarely make maps, or label the ones I do and I never publish to the web in any event.
Perhaps it is just another layer of complexity to add to things we have to remember.
If you publish a map service the display field can be queried using the parameter "text=<value>". For example, a map service for parcel data where the display field is set to PID (parcel ID):
Otherwise you will need to use the parameter "where=<SQL statement>" which is a little more complicated and longer. For example:
Both queries will get you the very same response. If most of your searches are going to be on the Parcel ID (PID) field, which is very likely on a map service for parcel data, it makes things simpler to have the display field set to PID.