Like most modern cars the BMW control arm is the support part for the suspension of your vehicle's connection to the front wheels. Unlike the A-frames of the past, the control arm is new since the introduction of independent front suspension on BMW models.
The configuration of this suspension part for your specific model of BMW is made to fit and help control the backward and frontward movement of the wheel. One of the leading reasons for the change in type of suspension used on the front of vehicles to include control arms and MacPherson struts is to reduce the play of the wheel from 2% to 1% from the old A-frame configuration that was used for most of the last century by car manufacturers.
The control arms built for use on a BMW include many different shapes and sizes. This is a make and model specific part and is not made to interchange with other vehicles. There are also many different shapes, length and attaching configurations they are used in. This includes ends that are both simple polymer bushings with a hole for the placement of a holding bolt and ball joints with a stud used for mounting the arm to the frame or strut.
The BMW control arm is built in many different configurations of the arm or bar itself. Most of them are curved so they will fit on the confined space on the front of your vehicle. This is an important part of your suspension and knowing what they look like is part of being a car owner. Because of this if your mechanic informs you a control arm needs to be replaced, ask for the old part. This will make it possible for you to not only inspect the old one, but allow you to get to know just what it looks like for future reference.