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

For more detail on how movements are made click here.