T0 to T4 are terms used in the Watch Industry to describe the different stages of assembly of a wrist watch.
T zero (T0) pre-assembly of bridges, main-plates and small components.
T one (T1) assembly of the movement and final adjustments
T two (T2) casing up of the assembled movement with the dial, hands and placed in the case.
T three (T3) adding the bracelet or strap, and buckle, presentation box and certificates etc