Em unit is the following. The unit is the width of the widest character in the font that you are using. Typically it is an M or W. The two used Em items are the Em Space and the Em dash. Both are the width of an M. Halft of the Em is called an En space or dash.
En Dashes are used for dashed in numbers like your telephone number. The Em dash is used to break a sentance. You can type this (--) in word and it will auto correct to am Em dash.
I have worked in the Printing indistry since 1994 and I have never come across anyone measuring the margin in Em spaces. I have seen millimeters, centimeters, points (72 per inch), Picas (12 points to a pica), and Inches used. Typically Picas are used for line lengths. A typography rule of thumb is that the line legth should be about two times (in Picas) the font size (points). Example. If you are using 12 point type, your line lengths should be around 24 Picas. This makes it easlier to read. Long lines of type are harder for the eye to follow.
Okay . . . again, I have never seen anyone use Em spaces to set up a margin. I would use an Em space for an endent rather than using the tab.