When importing layer symbology in ArcMap, choosing the 'Just the symbols' option causes different behavior depending on what type of symbology you are working with.
You can import layer symbology in ArcMap by clicking on the Import button on the Layer Symbology dialog box on the Symbology tab. You can import symbology from three sources:
1. An ArcGIS layer file (.lyr);
2. A layer in the current map;
3. An ArcView 3x legend file (.avl).
The 'Just the symbols' option is not always enabled. It is only enabled when the following two conditions are met:
1. The source symbology and the target symbology are of the same type (e.g both are Unique Values symbology, or both are Graduated Colors Symbology.
2. The source and target symbology have the same number of classes. You can think of a class as the same as a symbol (a class actually contains a symbol along with other things like a value and a label).
How 'Just the symbols' works if you are working with Unique Values symbology:
ArcMap looks at each value in the list of values in the target layer, and looks for a match in the source symbology. If it finds a match, that symbol is imported. If it does not find a match, that class stays intact in the target layer.
How 'Just the symbols' works if you are working with Graduated Color (or Graduated symbol) symbology:
ArcMap simply runs through all of the classes and imports each symbol from the corresponding class in the source symbology. Matching is based simply on the order of the classes.