"Magnetic flux density" is also known as the magnetic field,

The SI unit for this is the Tesla, written as T.

Magnetic flux density is not "also known as the magnetic field". It describes the intensity of a magnetic field.

The magnetic field used in machines is quantified in terms of its flux density (symbol: B), expressed in teslas. The flux density is established by the magnetic field strength (H), expressed in amperes per metre, set up in the field windings.

As the magnetic field strength increases, the flux density increases until it reaches saturation. This is the point when the magnetic domains within the magnetic circuit are all aligned. At this point, any further increase in magnetic field strength will fail to increase the flux density.

So saturation of the magnetic circuit limits the flux density of the field.