Must confess that I also had some trouble wading through the threads on this subject. But I believe it comes down to this:
Say you are flying more than one aircraft between two airports (i.e., you have more than one route between the same pair of airports airports). You can have at most two of those routes with frequencies that are not whole numbers, i.e., do end in ".5"
Example:
Route 1 - AAA to BBB - Freq 1.5
Route 2 - AAA to BBB - Freq 0.5
Route 3 - BBB to AAA - Freq 2.5
In this case you would be connecting AAA and BBB with three ".5" routes, thus a violation of the rule. If, for instance, route 3 was freq 3.0 instead of 2.5, it woul be alright.