PXIe-6612 and PXIe-6614 have similar specifications except for the fact that PXIe-6614 has an on board high precision OCXO. You can utilize this on board OCXO in your measurement/generation applications in the following ways:
1 - Inserting PXIe-6614 in System Timing Slot of your PXIe chassis - If your PXIe chassis has a VCXO for the onboard clock, you can insert the PXIe-6614 in System Timing Slot of your chassis which makes its OCXO 10 MHz Reference available to all other peripheral modules automatically by driving the 10 MHz OCXO clock onto the chassis PXI_Clk10_In. This will override the chassis built in oscillator, which transfers the accuracy and stability of the OCXO to the PXI_Clk10 and PXIe_Clk100. This will translate to high measurement/generation accuracy by the peripheral modules using this highly stable and accurate reference clock.
2 - Inserting PXIe-6614 in Peripheral Slot of your PXIe chassis - By inserting the PXIe-6614 in peripheral slot of your chassis, you can still utilize the high precision 10 MHz onboard reference as the counter SOURCE and GATE for highly accurate counter measurement and pulse generation. This can be achieved by routing the 10 MHz Reference to the CtrnSource using the DAQmx Channel Property Node:
You can also refer to the below hardware block diagram to understand the difference between inserting PXIe-6614 in System Timing Slot and Peripheral Slot of a PXIe chassis -