There is a fundamental difference between a SI unit and an absolute unit. An absolute unit (usually known also as a derived unit) is a physical quantity that is defined starting from some basic (and fundamental) units through a physical law. Depending on the system from which the starting fundamental units belong, the absolute unit can be a part of the SI unit or not.
The basic starting units in SI are Meter (for physical quantity length), Second (for physical quantity time), Ampere (for the physical quantity current intensity), Kg (for the physical quantity mass), mole (quantity of substance), Kelvin (temperature).
For example the Newton is an absolute unit (derived unit) for the Force that belongs to the SI system being equal with
while Dyne is an absolute unit (derived unit) for the Force in the CGS system
`1 Dyn =1g*(1cm)/(1s^2)`
There are cases when an absolute unit is regarded only as a derived unit from fundamental units of CGS system.