Your layer contains a simple line symbol with a line style other than solid. The display of this symbol in a basemap layer or map service will potentially differ from your standard ArcMap display.
Solutions
- Do nothing. Accept the new symbol drawing as is.
- Alternatively, you can change the line symbol to a cartographic line symbol with a similar dash pattern.
More information
This warning message is telling you the simple line symbol may appear differently in a basemap layer or a map service. This is because this type of symbol will draw as a solid style at a size larger than 1 point in ArcMap's display. In general, cartographic line symbols offer more control and better appearance than simple line symbols when drawing with patterns.