When your air conditioning system is malfunctioning, it is often due to a lack of refrigerant.  If this is the case, in order to get it back so that is operates efficiently, you will usually have two choices.  You can either opt to top off the system or you can evacuate and recharge it. 

What is a top off?

When you top off your air condition system, you are simply charging refrigerant into the system.  This is the less costly of the two options.  However, because the technician really has no way of knowing the precise amount of refrigerant that your system needs, his previous experience is usually relied on.  This could mean, though, that either not enough or too much refrigerant is charged into your vehicle.  This will definitely affect how your vehicle operates.

What is evacuating and recharging?

The option of evacuation and recharge does much more than the top off.  The left over refrigerant, if any, is first removed from the air condition system.  Then it is recycled in order to make sure the impurities are taken out of it.  It is then recharged back into your vehicle.  Because the technician has taken the refrigerant that was in your system out, he will know the amount needed to add in order to bring it to its proper level. 

Although the second option is definitely more thorough, either of these options will work.  If you are on a tight budget and the only service you can afford is a top off, go that route.  Otherwise, spend the extra money to ensure your air condition system receives a thorough servicing.