The detector is described as cylinders with a common axis, parallel to the magnetic field, and as planes perpendicular to the common axis. The common axis is referred to, for obvious reasons, as the z-axis. The cylinders are described by their radius and minimum and maximum extent in z (absolute value). The planes are described by their position in z (still absolute value) and their minimum an maximum radius. Hence the detector is rotationally symmetric around z-axis, and mirror symmetric around the x-y plane. In addition, the thickness in radiation lengths, and the type and precision of measurements are also given. Each cylinder or plane can be divided in repeating sectors of measuring and non-measuring parts, so that eg. blind sector-boundaries between TPC sectors or over-lapping silicon microvertex detectors can be simulated (see examples bellow).
Cylindrical and plane calorimeters can be specified in a similar fashion. The geometry is given in the same way, while the energy resolution is given by two parameters, and the shower axis measurement precision is given by four parameters. Note that if the detector includes tracking detectors outside the calorimeters (eg. muon-chambers), the material in the calorimeters should be given in the tracking detector section as well, in order to get the multiple-scattering description.
In a future release, also tracking detectors of a conical shape (ie. truncated cones with the z-axis as the symmetry axis) will be possible to specify. This will make it possible to describe tracking detectors that points towards the center (or any other point on the z-axis).