In the seated position, knees bent, you are working the soleus (the lower part of the calf). You can also work the soleus by going all the way down so your heels are close to or on the floor, then come back to make them parallel to the floor while doing standing calf raises. The gastrocnemius is the upper part of the calf that sticks out. You work those when you point your toes. You can hit your soleus or gastrocnemius on the same machine most of the time. One thing you can do if you don't have a seated calf machine, but insist that you want to do seated calf raises, is put two dumbells on the ground, sit on a bench and put your toes on the dumbells. You can either put a dumbell or some plates on your knees and do toe raises like that.