"kindly explain to me also the diff of Wet leg and dry leg."

Wet legs are used on differential pressure measurement where both sides are filled with liquid. Exception is level measurement in a pressurised vessel (boilers). Dry legs are used where the low side is exposed to atmosphere (using a DP transmitter for level measurement in a open tank.

For the rest of your question, the answer is buried in here:

Link

It's a little bit longer than I'd like to summarize, but if you take a quick look at that .pdf, the information is laid out in an easy-to-read fashion.

Obvious when you look...

Another poster inadvertently referred you to a flowmeter site, so just a few words...

The tapping point on the drum for the 'dry' leg is always above the liquid level. The tapping for the 'wet' leg is below the liquid level. The transmitter senses the difference in pressure between them, but since the lower point is filled with liquid, the weight of the liquid must be compensated for in the transmitter calibration.

'Knockout' means there are two different fluids present in the drum, so the SG of the fluid in the wet leg can change, and affect the accuracy of the level reading (important since the output signal is generally used to actuate a control valve).

An inert fluid eg glycol, is used to fill the 'wet' leg to ensure that the SG (and therefore the output signal) remain constant. This fluid does become contaminated by the process fluid over time, so routinely requires replacement / topping-up.

The 'dry leg' fills with process vapours over time so requires to be drained periodically.