T.Seal helps enhance the final results you get after the design is transferred on the fabric and T.Seal is a textured sheet that really pushes the graphic into the fabric of the shirt, creating more of a matte finish; T.Seal can also help with durability.
You can see link below for more details -
iColor T. Seal Finishing Sheet (Lasts Up To 500 Presses) A3 Size | DTG PRO