Clarity, warm tone and high output.
John Petrucci has had a lot of recent experience playing the D Sonic on his 6-string guitars, so when he wanted the same sound from his 7-string, DiMarzio couldn't say no. The D Sonic combination of clean and heavy that works so well with high-gain amps for both chords and soloing makes a great choice for a 7-string bridge pickup.
The DP706 guitar pickup is recommended for bridge position.
As with the 6-string D Sonic, the character of the sound can be altered by reversing the installation direction. This can be particularly important because of the effect it will have on the low B string. With the solid bar polepiece towards the neck, the low B will be fat and heavy. With the bar towards the bridge, the sound will be tighter with more attack. The solid bar coil is also warmer-sounding, so two different single-coil sounds are available with coil-splitting.