Grouping Symbolic Categories
Categorical data often has more categories than desirable for a given map. To produce a simpler map, selected categories can be combined (or grouped). This process is sometimes referred to a 'collapsing' the range of categories.
By default, ArcGIS uses a single symbol for all polygon features. To change symbology, open the layer's Properties sheet.
In the Layer Property sheet, choose the Symbology tab. For categorical data, choose Unique Values (under the Categories heading), then select the Field that will be used to categorize the data. Clicking Add All Values will do just that. Unchecking the box beside <all other values> will exclude data that has no value for the chosen field. Click OK to apply this symbology.
Note that there may be many (often too many) categories...
To group some of these categories, return to the Layer Properties
Using the shift key, multiple categories (values from the field) can be selected. Values that are for similar types of features can be combined (this requires an understanding of the data). In the example below, C (Commercial), C/I (Commercial/Industrial), C/I/RM (Commercial/Industrial/Residential Multiple) and GC (General Commercial) are selected. Having selected two or more categories, Right-click and choose Group Values.
Having Grouped these categories, enter an appropriate Label for the group (e.g., Commercial)
Other data types can be likewise grouped...
Other (non-Grouped) categories may need to also be Labeled to be consistent with the Labels of the Grouped categories
And more appropriate colors can be chosen to 'fit' with the categories...
The result is a much simpler 'Legend' for the layer...
Note also that there are many interior lines (between adjacent Grouped polygons) that can be somewhat distracting.
Returning to the Layer Properties sheet and the Symbology tab, Right-click and choose Properties for All Symbols...
Now the Outline Width can be reduced (or even set to 0 for no line), and/or the Outline Color can be set to a light gray (or even set to No Color for no line)
In the example below the categories have no outline
Note how this further 'cleans up' the data, producing a simpler map and strengthening the association of the grouped categories