What is the same about the em categories? what is different?

All 3 are CSS unit sizes which we can use for margins, fonts, borders etc.

Exp

CSS has four different unit sizes. These are:

Pixels (px)

Points (pt)

Ems (em)

Percentages(%)

These units are divided into two different groups. They are fixed and relative.

Pixels and points are fixed , whereas em and percentages are relative unit sizes. Relative unit sizes are good when creating scalable layouts.

Ems (em):

An em is a CSS unit that measures the size of a font. Originally, the em was equal to the width of the capital letter M, which is where its name originated.

It stands for "emphemeral unit" which is relative to the current font size.

The "em" is a scalable unit that  is used in web document media. Ems have mobile-device-friendly nature.

Pixels (px):

Pixels are fixed-size units that are used in screen media. One pixel is equal to one dot in computer. The problem with pixel unit is that it is not relative .

Points (pt):

Point values are only for print. A point is a unit of measurement use for real-life ink on paper. Generally, 72pts= 1 inch which is one real-life inch like on  a ruler. Point  is not recommended to use in web pages.

Generally, 1em=16px=12pt=100% if the body size is 100%.

Relative unit sized fonts change and fixed unit sized fonts remain the same.

For example,

body { font-size: 100%}

p{font-size: 16px}

because if the tool that you are using is not balanced it there would be false mass.