In the first instance evacuating refers to the removal of the refrigerant so that the system can be disassembled.
In the final act the system must be placed in a vacuum strong enough to lower the boiling point of water to below ambient temperature. The point being, that just sucking on a bottle of water won't remove the water, but reducing the boiling point till the water boils and then sucking out the vapor will get rid of the water. The level of vacuum necessary is defined by the temp of the day/refrigerant. The level of vacuum is probably only achieved in 10% of all evacuations as it requires vacuum pump oil to be changed after only a few events (something universally not done). The level of vacuum necessary can not be read with a standard gauge; a micron gauge is necessary.
Using the vacuum for leak checking is a good basic test but really isn't very accurate as there are great differences between the way varies components leak under pressure or vacuum. Vacuum can cause a leaking shaft seal to not leak (for instance).
Bosch Master, ASE Master, L1
33 years MB technician