You're not exactly correct in your thinking of draft control and position control.
Mixed control is basically what it sounds like. It's a "mix" of draft control and position control.
Position control just basically sets the position of the implement without any consideration of the drag placed on it. It can be "in ground" or "above ground". Good for mowing and such.
Draft control will change the position of the implement based on the drag experienced through the toplink. If you're plowing, the implement has a tendency to go deeper and deeper. The limiting factor would be either the implement totally buried or the tractor losing traction or pull. That's where draft control comes into play. It would
automatically raise the implement when the drag increases so as to then decrease the drag, or pull, experienced by the toplink. You can set your draft control on "sensitive" or "insensitive" and anywhere in between. If it is more in the sensitive setting it has a tendency to raise the implement more readily with minimal increase in drag. If you're in heavy soil you don't want the sensitive setting because it's likely the implement will never get deep enough. But what if you're in heavy soil and you have the draft control set relatively insensitive. Then suddenly the soil becomes lighter. The implement suddenly becomes buried much deeper with that draft control setting. This is where "Mixed Control" comes into play. You set the sensitivity on the draft control lever and the position
DEPTH LIMIT on the position control lever. Mixed control is basically draft control with a maximum depth position set. The 3ph height may never get to that position, based on the draft control settings, but if it does, it can't go deeper than that. Would be ideal for mixed soil conditions. If you're plowing along in draft control and then you noticed the implement getting too deep you could always pull back on the position control lever until the implement raises to a desired height. Now you're in "mixed control"! Hope that rambling makes sense.
There's also a good explanation on
TractorSmart.com. Just click on [INFO] and then click [TRACTOR THREE POINT LIFT TYPES].