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?
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.