12.3.5 Effective Length
The effective length of a mobile supporting structure is the distance the lifting appliance and trolley may travel along the runway between the end (travel) stops.
Overall length is the distance between the extremities of the structure measured along the line of the runway (longitudinally) and represents the minimum width through which the mobile supporting structure may pass when an allowance is added for clearance.

Figure 12.3.5-1 Length measurements