1. Insulation: the material used to cover the copper pipe, usually rubber foam, which helps to absorb water droplets formed due to condensation on the copper pipes when the air-conditioner is operating. There are generally 2 types of insulation: Class 0 and Class 1. An inferior quality will result in a higher condensation rate and cause leakages. ⅜ inches is usually recommended for the piping in trunking (plastic casing that encases the copper pipes, PVC drainage pipes and wiring) while ½ inches is preferred for piping that will be hidden by false ceilings or concealed in walls.
2. Grade (or gauge): thickness of the copper pipe, generally a thicker copper pipe is able to withstand a higher operating pressure. The grade varies from the lowest SWG (standard working gauge) or G25 (0.51mm) to G21 (thickest). For HDB owners, the G23 (0.61mm) is usually recommended. If you have concealed piping, then G22 (0.71mm) is a better option. Another thing you should take note is the bending of the copper piping. To lower the odds of condensation or poor refrigerant flow (which will make the air-conditioner less cold), make sure that the piping is bent with a proper pipe bender (thus ensuring a pipe bent of about 90 degrees).