Either 'phase' or 'depot' maintenance. But from the pix, am guessing phase maintenance.
Unscheduled maintenance is always because of things that malfunction from flying. An attitude displacement indicator (ADI) got its 'ball' stuck, then the ADI is replaced on landing is an example.
Phase maintenance is based upon time, or flying hours passed. The aircraft is turned over to a specific unit trained to perform detailed inspection and R/R of anything that shows certain signs of structural stresses, out of tolerance wear, corrosion, etc.
Depot level maintenance is where the aircraft is returned to either the manufacturer or a third party maintenance outfit certified by the manufacturer to literally open and expose the aircraft down to the bare frame. Components are removed and tested, no matter what condition or age, and replace if necessary. Paint is removed and re-applied, inside and out.
From the pix, fuel is removed for safety reasons, but hydraulic fluids are kept. The jet is on jacks, main gears are up but nose gear is down, indicating gear maintenance part of the phase inspection period. Not sure why the training missile is attached, unless it is used for troubleshooting purposes. All flight control surfaces are leveled, but no external hydraulics hoses are visible, so the flight control surfaces are rigged. Absent hydraulics pressure, flight control surfaces usually droops from their weight. Canopy is removed, meaning the Egress shop has their hands on the jet. Those white stripes topside are tapes holding panels in place, so most likely those panels are fuel tanks access. Fuel tanks comes in all shapes and sizes and they usually wraps around the engines. Hard to tell if the jet is at the beginning or near the end of phase but since there seems to be an engine cradle, that could mean the jet is being prepped to have its engine removed and that mean the jet is at the front half of phase.
