Difference between units of measurement (when to use each)

[Lorela5FF4]

Hello, I have a question. 
When to use each unit of measurement in Illustrator, (I have read  that for digital, px is used/ while for print is used pt). 
How about the other units? picas/inches/millimeters/centimeters? 

[Doug A Roberts · Accepted solution]

A pica is a unit used in typesetting, rarely in other contexts. It is simply 12 points.

You would use metric or imperial units if your media is commonly specified that way in your country. For example, in Europe, you would commonly use ISO-defined paper sizes in millimetres. In the USA, sizes in inches are more common.

A pixel is the odd one out, not being an actual unit of measurement (a pixel's 'size' is arbitrary). Within Illustrator, pixels are treated as equivalent to points.

[Mohammad.Harb · Accepted solution]

For anything will be viewed on digital screens ( Laptop, desktop, smartphones, tablet, .. ) use pixels to specify the dimensions ( width + height )

for anything will be used for print, you can use any unit you are most comfortable with ( Inch, mm, cm )

One last note: pt is the default unit to specify the font size in All application.