synchronous and subsynchronous content of voltage and current 2

never heard the term subsynchronous before.

where did YOU hear it?

Can you copy the whole paragraph you took this word out of so there is more meat to work with here?

Do you mean "Subsynchronous Resonance"[SSR] phenomenon?
I saw –in the far past-a synchronous condenser[compensator] of 20/60 MVAR connected in medium voltage system through capacitors in order to delete the 40 MVA transformer reactance and to facilitate rapid compensation of a required reactive power on H.V. system.
A large induction motor connected in the Grid which stator winding was executed with q fractional [q=number of slots per pole and phase] produced a sub-harmonic EMF and a resonance was establish and the synchronous condenser was shut down. However, could be
other effects too. See for instance:

If you mean Synchronous reactance and Sub-transient reactance the attached might help explain.

The synchronous frequency is the fundamental frequency (usually 50 or 60 Hz) of your system. Sub-synchronous frequencies are those below the synchronous frequency. I work in a utility environment, the sub-synchronous frequencies that we see are usually due to generator governors or AVRs fighting, or resonances within the transmission system (often excited by faults).