Grout fills the joints, supports the tile edges and keeps water and dirt out. EN 13888 distinguishes cementitious grouts, CG, from reaction-resin grouts, RG.
Cementitious grouts are class CG1 or the improved CG2, which can carry W for reduced water absorption and A for high abrasion resistance. Resin grouts, usually epoxy, are impervious and resist chemicals; they are used in kitchens, pools and wherever hygiene is critical.