Units of length are two categories: absolute and relative.
Absolute units include:

• inches (in)
• centimeters (cm)
• millimeters (mm)
• points (pt)
• pica (pc)

In the terms of CSS’ specification: 1pt = 1/72in and 1pc = 12pt.
In the property of font-size specification of negative value in units of length, for example: 25cm is unacceptable.
Why Absolute? Because they represent a real value in the physical world – standards.
These units work with the mechanism of output, which has an actual physical size, for example: printing, we will use such units.
But those units have no meaning for the monitors, there is certain conditionality, but it's really just conditionality.
Relative units include:

• em (font-size)
• x-height (ex)
• px (pixels)