The JTAG interface is used for programming and debugging, but has no function during operation of an end-product. It is clocked and active during sleep if the On-chip Debugging (OCD) feature is enabled. The OCD and JTAG interface should therefore be disabled if it is not needed.
The OCD can be disabled in fuses, while the JTAG interface can be disabled both in fuses and in software. Disabling the JTAG in software ensures that the MCU can be reprogrammed because the JTAG interface is re-enabled upon RESET.
Alternatively, the PDI interface can be used for programming and debugging. In this case the JTAG interface may not be required at all, and may be disabled by fuses. The PDI interface also works in all sleep modes.
To minimize power consumption, disable the OCD and the JTAG interface.