I have a polygon layer whose color needs to represent certain categories. There are a couple of polygons that belong to two categories at once (in the attribute table, the relevant cells literally contain a value of "[category 1], [category 2]"). Is there a way of showing this on the map with a crosshatched fill for those polygons, i.e., alternating diagonal lines of either color filling the polygon? If not, what is the closest that one can get to that kind of symbology in ArcGIS 10.6? Thanks everyone for any advice on this.
You could create a single symbology that included both colours on alternating lines, but I would not do it that way.
For situations like this, if there are 4 or less categories, I would symbolise each category with a cross hatch of a different colour AND different direction (vertical, horizontal, diagonal, diagonal the other way). This way will work even if an area includes all 4 categories.
(For more than 4 categories, you'd have to come up with some other combination of symbologies that could not possibly obscure each other. Eg, You could do 8 categories, by offsetting another 4 by half the distance between the lines.)
Then, as @michael_stimson commented, use a separate layer for each category, with definition queries for each layer to include only the features that match that category (including those that also match oither categories).
As @michael_stimson commented, your definition queries could look like:
field LIKE '%[category 1]%'
field LIKE '%[category 2]%'
(Note that as a general rule of thumb, its usually better to avoid a data model that could have multiple values in a field. Other options could include using a many-to-many relationship, or having two copies of a feature that is in two categories. This way, you'd only need a single layer using the "Categories" -> "Unique values" option in the Symbology tab, which would be the preferred and easier way to symbolise categories. This would avoid the need for a separate layer for each category.)