The tool is configured to solve using the network's hierarchy when the maximum straight-line distance between the farthest incidents and facilities exceeds the given limit. A hierarchical analysis solves faster than a nonhierarchical one, but the results may not be optimal. This limit is often imposed to minimize the processing required to solve the analysis, particularly on a server.
If you require an exact solve rather than a hierarchical one, place all your facilities and incidents within the threshold straight-line distance specified in the warning. You may need to break up your analysis into smaller ones.
Although the analysis would use hierarchy regardless, you can avoid this message by checking Use Hierarchy in Analysis.