A cam sensor is used for true sequential injection. Generally considered an unnecessary complication except for OEM's who are rigid with concern about meeting emission requirements. Sequential systems fires injectors based on where each individual cylinder is at in its cycle. A batch-fired system will fire off fuel for a set of cylinders, 2 or 4 at a time usually in a V8.
For emissions a sequential system lets you tailor the fuel pulse delivery for each cylinder individually (also good for piggybacking on a cylinder deactivation system). Most sequential systems convert (within their programming) to a batch-fire system at some set rpm level.
Apparently you can add a cam position sensor to an Electromotive system to get a true sequential injection system, if you want. See page 20...
http://electromotive-inc.com/pdf_files/tec3.pdf