Titanium Grade 1-4 is pure Titanium, the other grades are alloys. Pure Titanium is used due to it's high corrosion resistance, the alloys because of the extremely high strength to weight ratio.

Grade 1 Pure Titanium, relatively low strength and high ducility. Plate heat exchangers
Grade 2 The pure titanium most used. The best combination of strength, ducility and weldability. Piping systems.
Grade 3 High strength Titanium, used for Matrix-plates in shell and tube heat exchangers.
Grade 5 The most manufactured titanium alloy. Exceedingly high strength. High heat resistance. Aerospace, subsea.
Grade 7 Superior corrosion resistance in reducing and oxiding enviroments. Chemical Industry.
Grade 9 Very high strength and corrosion resistance. Hydraulic piping, subsea.
Grade 11 Applications as for gr 7. Suitable for deep drawing.
Grade 12 Better heat resistance than pure Titanium. Applications as for grade 7 and 11. Shell and tube heat exchangers.