A variable frequency drive is an adjustable speed drive. Don't worry about it. It's not clear to anyone. A little about AC drives The main power components of an AC drive, have to be able to supply the required level of current and voltage in a form the motor can use. The controls have to be able to provide the user with necessary adjustments such as minimum and maximum speed settings, so that the drive can be adapted to the user's process.
Spare parts have to be available and the repair manual has to be readable.
It's nice if the drive can shut itself down when detecting either an internal or an external problem. The paradox facing drive manufacturers today is that as they make their drives easier to use, the amount of training with which they must provide their users increases. This is because as drives become easier to use they are purchased more and more by people of less and less technical capability. The real action in an AC variable frequency drive system is in the motor. This is really where it all happens.