Synchronous clear/ Asynchronous clear is the method about how to use Z phase. When the counter is zero-cleared, you should consider the states of A phase and B phase. Only according to the change of Z phase is wrong.
Synchronous clear exists only when the mode is 2-phase.
Asynchronous clear is the clear method when the mode is 2-phase, mono-phase or gate control.
Synchronous clear
Synchronous is, which the counter is zero-cleared when A phase rises with B phase being LOW and Z phase being valid.
When the logic of Z phase is set to LOW ACTIVE, it is cleared with Z phase being LOW. When the logic of Z phase is set to HIGH ACTIVE, it is cleared with Z phase being HIGH.
Asynchronous clear
Asynchronous is, which the counter is zero-cleared when Z phase becomes valid, irrespective of the state of A phase or B phase. When the logic of Z phase is set to LOW ACTIVE, it is cleared with Z phase being LOW. When the logic of Z phase is set to HIGH ACTIVE, it is cleared with Z phase being HIGH.