Boundary problems in stratigraphy
I believe what is being referred to here is the problem of determining where, in a continuous deposition sequence, a particular geologic time transition occurs.  If you have uniform depositional conditions you look for a transition in fossil communities that have been agreed on as marking the transition.  Life is not so simple and you could have a delta sequence overlaying a deep water deposit. Obviously the geologic aeon does not change everywhere at the contact between clay and sandstone and one must determine where in the clay the line is to be drawn and where in the sandstone.  On a larger scale a time boundary that, in the type section, starts or ends at an unconformity needs to be more precisely defined so that correlations across geologic provinces can be made.