A fish route or alpha route is defined as a highway having a planimetric shape resembling a fish or the greek letter α. These routes frequently occur in highway system interchanges where a route curves with a relatively small radius to pass over or under itself, often in conjunction with a larger, grade separated interchange. The idea is to support this configuration of highways in the Esri Roads and Highways location referencing extension without the need for a workaround solution.
Currently, the position of Esri is that the alpha routes are supported IF the introduces a small gap in the route where the intersection would occur at the grade separation. The position of the Roads and Highways users is that this workaround is crude, it doesn't meet our definition of support for the shape of these highway designs, and it results in other data processing challenges.