Compound Measurement:
Compound Measurement is a combination of two simple measurements.
Simple measurement is used to specify the quantity or unit price of a stock item in round numbers. (Approximate value)
Compound Measurement is used to specify the quantity or unit price of a stock item in decimal values. (Exact value)

Compound measurement can be created by specifying the relationship between two existing simple measurements.
Example:
simple measurements
bg
kgs

Compound measurement created from the above simple measurements:
bg of 25 kgs

In the above example, bg is considered as main measurement and kgs will be secondary measurement.

if you omit the measurement symbol, at the time of specifying quantity or unit price then main measurement will be taken automatically.