1. Tonnage is the amount of heat the air conditioner is capable of removing from the room in one hour. The mathematical unit for tonnage is BTU (British Thermal Units).
1 ton equals 12000 BTU. The higher the BTU, the greater is the cooling effect.
The tonnage of the AC should be decided by you according to your daily usage, size of the room, location, and climate of that place. So, choose wisely according to your needs.
2. Star Rating (Efficiency) was introduced by the Bureau of Energy Efficiency (BEE) India, with a range of 1 to 5 stars. This showcases how energy efficient, the electrical appliance is. More the stars, the greater the efficiency of your electrical appliance. Although, high star rated air conditioners are expensive than the lower ones. But, in long term, you are going to save a lot of electricity with a 5-star rated AC than 3-star ACs.
3. There are various modes of operation available in the air-conditioners-fan mode, cool and dry mode, energy saver mode, sleep mode, 6th sense cooling, heater, etc. Whatever AC suits all your choices, go for it.