It is an indicator that some part or some subassembly at a lower level has flexibility applied. This is helpful to detect when some item in a complex assembly does not match the default values.
For example, someone might, for bad reasons, change the length of a fastener using flexibility. This would mean that the part no longer matched the drawing of the part and the part that manufacturing purchasing would buy would not be the size as shown in the assembly. If this is a complex assembly it might take looking at every subassembly to see if any of the parts had been changed in a way they should not have been on the slight chance that it happened.
With the flag a user can see the subassemblies that have items that don't have flexibility, saving having to inspect them.
